The online book TCP/IP Illustrated Vol 1 is available online. Note that this is an older book dating from 1994, and is also not for the faint of heart as it skips past the high-level view of TCP/IP and actually digs right down into the nuts and bolts that make it work, in conjunction with real-world illustrative examples (hence the book name!). I should read this volume at some point, but maybe not quite yet until I get some more sniffing experience under my belt.