1. The Bible. It encourages me to live in a way that's also good for others, especially when I'm feeling selfish and cynical, and it teaches me how to interact with them in a healthy way (e.g. the book of Proverbs).
2. The Elements of Style. I always enjoyed writing, but at first school taught me to write in a flowery, longwinded way. This was the book that cracked the code for me to good writing. It dispelled a lot of self-serving and ultimately self-defeating habits and paved the way to clean, helpful English. When I finally got into programming in my late twenties, I found that many of the same principles make good code.
3. The Visual Display of Quantitative Information. This is like the Elements of Style but for graphs. Again, it encouraged me to cut through the hype and deliver the content as clearly and succinctly as possible --- to serve the reader, not stroke my ego.