Humble Book Bundle: Unix
Just to be clear the prices are not per book. I don't know if anyone else here stood dangerously long in that misunderstanding, almost preventing them from pulling the trigger. It's $1 for all the books on the first tier, $8 for all the books on the first and second tiers, or $15 for all of the books on the page. At first I was like, well maybe I'll get this book, this book, and that book. But then it was a no-brainer. In fact on the recommendation of a couple of posts here for DNS & Bind, I bought the whole hog.---  $1:
  Unix in a Nutshell (4th ed.)
  sed & awk (2nd ed.)
  lex and yacc (2nd ed.)
  Learning the bash Shell (3rd ed.)
  Linux Pocket Guide (3rd ed.)

  $8: (all of the above, plus)
  bash Cookbook
  Classic Shell Scripting
  Learning GNU Emacs (3rd ed.)
  Unix Power Tools
  Learning the vi and Vim Editors (7th ed.)
  Bash Pocket Reference (2nd ed.)
  Learning Unix for OS X (2nd ed.)

  $15: (all of the above, plus)
  Essential System Administration (3rd ed.)
  TCP/IP Network Administration (3rd ed.)
  DNS and BIND (5th ed.)
  Network Troubleshooting Tools