The Success Principles by Jack Canfield
This book written by an author of 'Chicken Soup for the Soul' is an excellent synopsis of the majority of self improvement ideas. Although it rarely touches on NLP, the book is large enough to include most ideas around the field of self help.
The principles are sound and well known but it's an excellent chance to buy one book to familiarise yourself with all of them at once. After reading this book there will be very little left in the self help field that will seem very new.
I recommend this book.