Understanding the Linux Kernel by Bovet and Cesati is a pretty good book that provides in-depth implementation information on all aspects of the linux kernel. The only thing you should know is it covers the 2.4 kernel.
Linux Kernel Development by Robert Love is a similar type book which covers 2.6 specifics. I haven't read the new addition that just came out on Jan 12, 2005 but the old addition, while a great book, didn't go in nearly as much detail as the Bovet and Cesati's book does.
Since the Linux TCP/IP stack hasn't really changed in quite a while you'll probably get the most out off the 2.4 book.
Also check the Documentation folder in the kernel source itself, and it certainly doesn't hurt to open up the code and read the comments once you have done some of the upfront reading on the basics. I haven't done much work with the TCP/IP section myself, but many sections of the kernel are surprisingly well documented. Most functions are very small and easy to understand as well.
