You want to become an even better developer? Then you must read these books. If you have already read them. Read them again.
Design Patterns. Elements of Reusable Object-Oriented Software: This is the standard book on patterns and patterns are an extremely important piece of vocabulary for every developer. Not to impress coworkers with your knowledge, but to be able to think about bigger concepts than loops and conditionals. It describes each pattern with its purpose, it's applicability and an example. Make sure to read a couple of blog posts about each pattern, to make sure you avoid some of the more common mistakes when applying them.
Clean Code: A Handbook of Agile Software Craftsmanship: You want to be proud of your code? You want to be able to go back to your code in a year without swearing? This is the book for you. The author Robert C. Martin (also known as Uncle Bob) is extremely aggressive on what he is accepting as clean. This book really changed the way I look at code, and of course the way I write code. Make sure to really understand the lengthy examples. It is tedious, but it pays.
The Pragmatic Programmer. From Journeyman to Master: In Software Development the professionals often follow practices which the professionals them selfs aren't aware of. This book lists and explains many of these practices as well as some obvious once. While this sounds trivial the resulting book is a great source of small and big practices to acquire as a habit.
For the German readers of my blog: Read the english originals. German translations of computer books are horrible on average.