Code Complete: 2nd Edition
If you were only able to choose one book to make you a better programmer, that book would be Code Complete. There is a reason it’s the highest voted answer to the StackOverflow questions What is the single most influential book every programmer should read? and What development book made the most impact on you as a developer? as well as many others. Notice that second place for both questions is Pragmatic Programmer, you should go read that too, after Code Complete.
There really isn’t anything I can say about Code Complete to describe how truly awesome it is. From your future self and all of the people you work with please buy it and read it from cover to cover. I’m not kidding, you might think you know everything it covers, and if you’ve been programming for a few years you might even be right, but you won’t have thought about each of the topics explicitly and in the detail covered by Code Complete.