It's a great introduction to mathematical logic and the philosophy of mathematics. Despite being written as a textbook (so far I know), it can be read like a normal non-fiction book, and you don't get that from every textbook. Except it gets really boring and confusing at times, but maybe that's just something on my end. Chapter 7 (Gödel's Incompleteness Theorem and Computability) is especially worth a read!

Also, did I mention that this book is really re-readable?