The Richest Man in Babylon, by George S. Clason. The message of this 1926 book is that rich people are rich because they save their money and don't get into debt.
Your Money or Your Life, by Vicki Robin and Joe Dominguez. This book will change your relationship with money.
The Little Book of Common Sense Investing, by Jack Bogle. This book is about index funds. Jack Bogle founded Vanguard and created index funds.