If you code in python
If you code in python... you should own this book.
Most concise and readable language implementation reference I've ever read. Doesn't get too far down into the details of the interpreter, while giving all the details an advanced programmer will need.
Pros: It takes four hours to read and will teach you things you weren't aware existed in the python language (well, unless you develop for cpython or something).
Cons: You might need a degree in CS to be able to read this book, it assumes a lot of knowledge. On the other hand, it's cheap, so if you don't understand it you didn't waste a lot of money.
