Convinced me to leave Java and start doing Ruby instead. Did a lot more programming languages since then.
* Javascript the Good Parts, by Douglas Crockford
Taught me how to write better js. Of course since then the language has changed so much that this little booklet is completely obsolete, but for a brief period, this book was absolutely essential.
It's hard to single out a third book. No other books come close to these two. Most books are too thick and wordy and boring, and I'm not a fast reader. I'd love to list one of the books about design patterns here, but I never finished any of them, so I think I'm going with:
* Beginning Scala
I haven't actually done any big projects in Scala, but it was captivating, showed me how beautiful a programming language could be (but also how ugly, in later chapters), and changed how I think about programming languages. I still regret never having actually begun anything in Scala, although I still fear the complexity of its type system. The world needs a cleaner Scala.