"Thinking in Systems: A Primer" by Donella Meadows changed how I approached designing/troubleshooting software systems as well as changed how I think about political policy decisions and their results.
"Object Thinking" by David West dramatically altered how I approach designing OO systems. I especially liked the chapter(s) where he used different real-world metaphors for designing systems. For example, asynchronous communication (email) is often more appropriate than synchronous communication (calling someone on the phone). Delegation of tasks without "micromanaging" (i.e tell don't ask).
"Ever Wonder Why?" by Thomas Sowell gave me an insight into some of the underpinnings of Conservative thought. I'd never had the opportunity to hear any of the arguments he brings up in college or in my own liberal social groups.