I love reading books so this is really difficult to pick the best. Rich dad, poor dad is good, The 7 habits of highly effective people is good, the How to win friends and influence people is definitely a classic. But to me, the best of the best book maybe The 4 hour workweek by Tim Ferris.
This book is so practical, even though it was written like 10 years ago, the concept still apply today. It teaches you how to become a new rich. Versus the traditional rich, new rich is financial independent, work less, capable to enjoy life. It paints out a picture that people can actually works only 4 hours a week but get 10x more income. The book has provided a lot of practical approaches and tools that you can find online and start your business, with very little initial investments. Tim Ferris is a life hacker, you need to read his books!