Upvoted, and I would also say the Book of Mormon, with the Bible. The BoM reinforces the Bible's teachings powerfully, and as I age and read again, sometimes in other languages, and some scholarly things about it, they both just keep getting better and better. Including in their effect on me. You can get a copy of either one for free. Edit: Additionally, some which have been mentioned or not: - a couple of biographies of Lincoln (one is free on gutenberg.org, the other more recent by Doris Kearns Goodwin), and one of Washington whose name I forget now, but there are good ones out there. - the 7 Habits book (Covey), and How to Win Friends and Influence People by Carnegie. - Refactoring ..., by Martin Fowler. - some thick Scheme programming book from college (sorry, it's downstairs). And "The Little Schemer", also about Scheme (one of the authors, Friedman, was also our instructor). - Since Cumorah, by Hugh Nibley (maybe dated now, but there is a lot of related work). Really got me interested in related topics.
Also recommended in this reply