The Slight Edge revolves around a simple idea: the simple things we do each day, repeated over time will lead to success (or failure) - eat healthily for a long period of time and you will be healthy. Eat junk for a long period of time and you will get very unhealthy, even though it won't show immediately; this is the Slight Edge and it can work for or against you.
The second part of the book is an effort at explaining how to apply the Slight Edge to your life as well as other advices you can find in other self help books such as writing down your life goals or "plan, do, review" (i.e. iterate through trial and error)
Even though the book wasn't a bad read, several months after reading it, I think it is not great either. The author gives a few examples on how to apply the Slight Edge. The main one, repeated several times in the book, is reading 10 pages of a good book every day. If you are already an avid reader, this isn't much. I find myself reading this book, feeling good about it but in practical terms, I failed to apply it or even figure out how to apply the Slight Edge to my life goals. The "Slight Edge" is a "feel good" book rather than a good book. The book, like many self-help books, has also a tendency to make you feel guilty if you do the wrong things or think the wrong way. Other books convinced me it is the wrong approach to make people do something.
Very few self-help books are worth the money and the time you spend reading them and sadly, this one is no exception. Instead, I would recommend reading books based on psychology and research. Two books I found useful and actually helped me: The "Now Habit: A Strategic Program for Overcoming Procrastination and Enjoying Guilt-free Play" by Neil Fiore and "Mastery" by Robert Greene.