The one book I found very helpful when I was studying in graduate school and have adopted to my pool game, as well as other aspects of life, is:
How Life Imitates Chess
by: Gary Kasparov
I found this book very rewarding in both school and other aspects of life, such as pool. I do find myself re-reading parts or even the whole book every couple of years. The main points to take away from it is that, in order to be successfull, you must have a solid routine. Any excuse saying, "Well, I'll just vary from the routine just this one day" is unacceptable he says, as that breaks the spirit, diligence, and focus that you are trying to achieve.
Another great part was when he was studying chess at a young age, his coach would turn on the radio/tv extremely loud, face it at his ears, then blow cigar smoke into his face while Gary was tryingn to figure out the next correct move. Gary's response to his coach on this sensory overload was that it was breaking his concentration. His coaches reply, "What does any of that have to do with chess?" and he has adopted this maxim ever since.
It's my #1 go to book when I need encouragement or begin breaking my good habits.
A second on my list is
Rational Living
by: Henry Churchill King
A long forgotten and lost book. I believe it was written in 1905, and I was lucky to find a copy from then at a local userd bookstore. The book is ungodly insightful and full of beautiful quotes and maxims to live by. This is another book I have sections with post-it notes in that I go to on a regular basis to remind myself of the true art of being rational and living to acheive optimal performance. If you can find it, buy it. It's a must read for anyone wanting to improve their mental state.