Other have mentioned The Mythical Man Month, and I'd say it's a great book. Sadly, more often than not, it's something I wish I could staple to managements foreheads. "read this now before our next planning session"
The Rspec book was a great book, that helped me fully embrace TDD, and this has led to more sleep filled nights than I had a right to before its consumption.
1984... This one terrified me so much, that I have to include it. I write code much more securely because of it.
Strange in a Strange Land. If only because I learned what GROK means.