I've read all of the books listed by Ayman, jldugger and packs, and I think they are all excellent suggestions. The TCP/IP Guide is more "How does it work", TCP/IP Illustrated Volume is more about the nuts and bolts of the protocol and, is somewhat dated (though still a must have classic for your bookshelf) - particularly with it's discussion of classful addressing, a topic which the TCP/IP Guide handles masterfully. TCP/IP guide is actually a very good practical intro to the protocol; in addition to providing a phenomenal history of CIDR, it also gives you an insane level of detail on PPP and other protocols not often gone into in much detail.
Tanenbaum is more about the holistic "What do networks consist of." and is an excellent addition to fill out your library. Comer is a classic which has also taught a lot of people in the networking profession practical TCP/IP.
So, those four books make for an excellent library on the topic of TCP/IP networking, and I would really like to suggest one more, which provides a fundamental discussion on networking protocols, referencing TCP/IP (among others) as a teaching aid to protocols:
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Interconnections is my "Desert Island" book - you can read, and re-read some of the pages and chapters and continue to get deeper insight into the whirling maze of protocols that you interact with. Highly recommended and I think an must-have for every complete bookshelf on computer networking.