"Whatever your mind can conceive and believe it can achieve." - Napoleon Hill
The above quote should have been the real title of the book, since Think and grow Rich is very misleading. The title is just a bait, it seems. Napoleon Hill neither really says in this book how to become rich, nor he was rich himself, so he's hardly the authority...
He was, however, a very successful "personal achievement" advisor. He was a personal consultant to Andrew Carnegie, and during his life interviewed hundreds of other highly successful people. In this book, he tries to share with the reader what's common with all those people.
Again, this book won't tell you how to become rich (no book will). If you want some motivation on real monetary issues, read "Poor dad, rich dad" (the first book, only). But it might tell you something about yourself, and will lead you to understand that you have everything you need to succeed inside yourself.
Frankly, if you already read some "personal success" books and liked them, you don't need this one - it won't tell you anything new.