Surprised TCP/IP Illustrated (Volume 1) has only been mentioned 6 times. It's been so helpful for me, so many times. Perhaps it's because most people haven't had writing a TCP stack as part of their day job, but it's such a fundamental technology I would have thought learning about it in depth would be suggested far more frequently.

Also, a proper first edition copy is really high quality with lovely thick paper. My copy of Volume 2 on the other hand is not of the same quality, both in content and physical properties.