TCP Illustrated; Volume 3 by W. Richard Stevens, but you don’t really need to dive into the details of TCP for Transactions, which turned out to have security flaws. Understanding of IP, TCP/IP and UDP/IP is a must.
The Design and Implementation of the 4.4BSD Operating System by Marshall Kirk McKusick
The UNIX Programming Environment by Brian Kernighan and Rob Pike
The Practice of System and Network Administration by Tom Limoncelli, Christina Hogan and Strata R Chalup