The Slight Edge is written by Jeff Olson, and the subtitle of the book is "Turning Simple Disciplines into Massive Success and Happiness." That covers very well the general theme. Be warned - I had tons of things highlighted after reading this book, so it may get a bit long.
If I had to sum up the main idea in my own words, I think it would be the small habits you develop are very powerful and set you on the path to great success or else to a mediocre life of struggle. From reading the book, but own opinion is that's part of why daily prayer and meditation are so powerful. And why it's dangerous to let the habit fall into disrepair. According to the book, only about 5% of people truly learn this lesson and apply it. I thought this book was very good, and I found it interesting how many of the ideas parallel the lessons in the Bible. It's also amazing to me how many of the same themes reappear in many of the leadership or self-improvement books that I read. Being vulnerable and taking risk, learning from failure, the power of your thoughts and attitude, the importance of lifelong learning, etc. The saying that I found most memorable that still comes to mind frequently is "Easy to do, easy not to do."

Some of my favorite passages from the book follow here:
"I call them simple daily disciplines. Simple productive actions, repeated consistently over time. That, in a nutshell, is the slight edge."
"How to do it is not the issue. If 'how to do it' were the answer, it'd be done. It's how you do the 'hows' that's most important."
"You've probably heard the expression, 'Failure is not an option.' Oh, really? Well, here's a reality check: failure had better be an option, because whether or not you consider it an option, it's going to happen! If you go through life with the philosophy that 'failure is not an option,' then you'll never have any good opportunities to learn."
"Success is the progressive realization of a worthy ideal. Successful people do what unsuccessful people are not willing to do. There is a natural progression to everything in life; plant, cultivate, harvest."
"The step we've lost touch with, the one where the real (though invisible) power lies, is the step of cultivating. And that step, unlike planting and harvesting, takes place only through the patient dimension of time."
"The things you do every single day, the things that don't look dramatic, that don't even look like they matter, do matter. That they not only make a difference - they make all the difference."
"The slight edge is relentless and cuts both ways; simple daily disciplines or simple errors in judgement, repeated consistently over time, make you or break you."
"The simple things that lead to success are all easy to do. But they're also just as easy not to do."
"The things that create success in the long run don't look like they're having any impact at all in the short run." "In fact, the difference between success and failure is so subtle, so mundane, that most people miss it. They may not realize they have a philosophy, but they do, and it goes like this: What I do right now doesn't really matter." "The truth is, what you do matters. What you do today matters. What you do every day matters."
"People don't consistently do those simple things for three reasons:
1. While they're easy to do, they are also easy not to do
2. You don't see any results at first.
3. They seem insignificant, like they don't matter. But they do."
"The secret of time is simply this: time is the force that magnifies those little, almost imperceptible, seemingly insignificant things you do every day into something titanic and unstoppable.
Consistently repeated daily actions + Time = Inconquerable results"
"Successful people form habits that feed their success, instead of habits that feed their failure."
"If I stay on this road long enough, I'll get the result I'm seeking. It's not a question of your mood or your feelings. And it's not a question of will power. It's a question of simply knowing."
"Slow down to go fast. In other words: you want big results? Good - then do the little things. Just do them consistently and persistently."
"Some more shocking news: your ship's not coming - it's already here. Docked and waiting."
"Believing in the 'big break' is worse than simply being futile. It's actually dangerous, because it can keep you from taking the actions you need to take to create the results you want."
"Once you absorb the slight edge way of being, you'll stop looking for that quantum leap - and start building it. You'll stop looking for the miracle, and start being the miracle."
"No success is immediate, no collapse is sudden. They are both the result of the slight edge accruing momentum over time."
"Greatness is always in the moment of the decision, and so is fate."
"Growth creates abundance, and that abundance in turn creates responsibility. The more success we have, the more greatness we step into and the more abundance we experience, the more responsibility we have to the world around us. Achieving greatness - in anything - isn't simply a matter of taking the steps that get you there. It's also a matter of being willing to pay the price. And that price is responsibility."
"There may be times when things seem to be on a steady, even keel. This is an illusion: in life, there is no such thing as staying in the same place. There are no straight lines; everything curves. If you're not increasing, you're decreasing."
"Life is a curve construction; time is its builder, and choice its master architect."
"The predominant state of mind displayed by those people on the failure curve is blame. The predominant state of mind displayed by those people on the success curve is responsibility."
"Negative and difficult things happen to all of us; most of them are mostly or completely out of our control. It's how we react, how we view those circumstances and conditions, that makes the difference between success and failure - and that is completely within our control."
"Take care what you think. Because what you think, multiplied by action plus time, will create what you get."
"One of the quickest and most direct routes to getting yourself up and onto the success curve is to get out of the past. Review the past, but only for the purpose of making a better plan. Review it, understand and take responsibility for the errors you've made, and use it as a tool to do differently in the future. And don't spend a great deal of time doing even that - the future is a far better tool than the past."
"Like ripples in a pond, even small successes in any one of these areas begins to affect all the others, too. Improve your health and you improve your relationships; work on your personal development and you have an impact on your career. Everything affects everything else."
"If you are having a hard time making progress in one area - say, in business - take action to make a small positive change in an unrelated area. Feeling successful in one area will provide you with renewed confidence and energy to continue on your journey of attaining that other big goal. Success in one area breeds success in every area. The key is, start somewhere."
"Mastery is not an exalted state that lies at the end of the path; it is a state of mind that lies at the very beginning. Mastery is in the act of setting your foot on the path, not in reaching its end."
"Settling for less, giving up on the power of baby steps and embracing failure, soon becomes a habit."
"You can gauge the limitations of a person's life by the size of the problems that get him or her down. You can measure the impact a person's life has by the size of the problems he or she solves."
"The people living on top, those who take responsibility, live a life that is in some ways uncomfortable. Successful people do what unsuccessful people are not willing to do, and that often means living outside the limits of one's comfort zone."
"Among high school graduates who do not go on to college, 58% - more than half - never read a book again. I mean, for the rest of their lives."
"The only way to have power is to do the thing. Just do it. Learn by doing."
"You can't excel based purely on knowledge learned through study; and you can't excel purely through knowledge gleaned through action. The two have to work together. You study, and then you do activity. The activity changes your frame of reference, and now you are in a place where you can learn more. Then you learn more, and it gives you more insight into what you experienced in your activity, so now you re-approach activity with more insight. And back and forth it goes."
"You, through the power of your own thoughts, are the most influential person in your life. Which means there is nobody more effective at undermining your success - and nobody more effective at supporting your success."
"The principal aim in self-investment is to train how you think and what you think."
"Most of your life - 99.99% - is made up of things you do on automatic pilot. Which means it's essential that you take charge of your automatic pilot's training."
"You are the combined average of the five people you associate with most - including the way you walk, talk, act, think, and dress. Your income, your accomplishments, even your values and philosophy will reflect them."
"Going too fast, or growing too fast, often puts the system's (or the person's) survival at risk. Faster can easily turn out to be slower."
"There's nothing inherently good about slowness, and it's just as possible to move too slowly as to move too quickly. The key word in the Aesop moral is not 'slow'. The key word here is steady. Steady wins the race. That's the truth of it. Because steady is what taps into the power of the slight edge."
"There's another reason once a day for a week is better than seven times in one day, once a week; the daily rhythm of the thing starts to change you. It becomes part of your routine, and as it does, it becomes part of who you are. That doesn't happen with a once-in-a-while, all-out effort."
"Being productive and being busy are not necessarily the same thing. Doing things won't create your success; doing the right things will. And if you're doing the wrong things, doing more of them won't increase your odds of success. It will only make you fail faster."
"By simply showing up you can rise above half of the population in any circumstance."
"The funk finds everyone. But here is what I've learned, and it has saved me and pulled me from the funk more than once: There is no way I can understand love if I haven't felt the hurt of loneliness. I can't know what good is and how good it feels, without knowing bad. I can't feel happy and content without feeling the funk. Life is ebb and flow."
"The key to outrageous success in any endeavor is to put in about 10,000 hours of practice."
"It's in that moment's decision, when nobody else is watching and no one will ever know, when your choice is so slight, so subtle, so insignificant...it's at that moment that you find out whether or not you have slight edge integrity."
"The point is simply to come up with a plan that will get you out of the starting gate. It's not even that your starting plan doesn't necessarily get you there - it for sure won't get you there, at least not the exact plan you conceive at first. Nobody has that degree of perfect precision in long-range planning, and there are too many variables and surprises along the way that will require adjustment to the plan."
"The power of a plan is not that it will get you there. The power of a plan is that it will get you started."
"Happy habits:
- Each morning, write down three things you're grateful for. Not the same three every day; find three new things to write about. That trains your brain to search your circumstances and hunt for the positive.
- Journal for two minutes a day about one positive experience you've had over the past twenty-four hours. Write down every detail you can remember; this causes your brain to literally re-experience the experience, which doubles its positive impact.
- Meditate daily. Nothing fancy; just stop all activity, relax, and watch your breath go in and out for two minutes. This trains your brain to focus where you want it to, and not get distracted by negativity in your environment.
- Do a random act of kindness over the course of each day. To make this simple, Shawn often recommends a specific act of kindness: at the start of each day, take two minutes to write an email to someone you know praising them or thanking them for something they did.
- Exercise for fifteen minutes daily. Simple cardio, even a brisk walk, has a powerful antidepressant impact, in many cases stronger (and more long-lasting) than an actual antidepressant!"