I recently came across a stack of free older books on network security:

  Cryptography and Network Security - William Stallings
  Network Security Foundations - Matthew Strebe
  Counter Hack - Ed Skoudis

I am fairly new to the world of network security, but as a developer am interested in learning more. I'm guessing much of the foundational stuff in these books is still mostly relevant but was wondering if anyone can suggest any newer up to date books in a similar vein?