It might not hurt to make sure you have your fundamentals straight first. TCP/IP Illustrated published by Addison-Wesley is a good source. It’s a three volume set and comes packaged with a poster that lays out the data structures for BSD sockets. Its only major drawback is that it doesn’t cover IPv6.
Dave Aitel of ImmunitySec authored a book entitled “The Hacker’s Handbook: The Strategy behind Breaking into and Defending Networks” It’s from 2003, but still very relevant in many respects. I highly recommend it if your practical network penetration testing experience is limited as it covers things from both internal and external perspectives.