TCP/IP Network Administration, by CraigHunt (O'Reilly & Associates, SebastopolCA, 2002), ISBN 0596002971. The modest title hides the fact that this book will be useful to a wider audience than just network administrators. It is a very good practical guide to TCP/IP written as a tutorial introduction.
Internet Core Protocols: The Definitive Guide by EricA.Hall (O'Reilly, 2000) It's a cover-to-cover read. The book's only defect is the reliance on a now-unavailable commercial tool. (The tool can be recreated for free using Ethereal, instead).
Unix Network Programming, by W.RichardStevens (Prentice Hall, NJ, 1990) The canonical guide to network programming.