Reddit mentions of sed & awk

Three esxi servers, each with more than one NIC, and one separate vcenter server is a great starting point. This allows you to tune DRS, HA, and even fault tolerance. Once you get that down, you'll want to be able to tune VMs properly to run most effectively in a virtual environment.

I enjoyed reading these books, though some are "dated" now, the contents are still very relevant. They won't get you anywhere in particular by themselves, but when you combine them with the self-teaching nature of sysadmins I've previously described, these will generously add to your toolset.

HA and DRS deepdive
Sed & Awk

Mastering Regular Expressions. I use rubular.com often.

Pro Puppet

Anything by Bruce Schneier is usually worth your time.

Though I no longer administer a large number of Windows machines, I am a huge fan of Mark Minasi. The Server 2003 book was super helpful in building and maintaining Windows Domains.

I have an old edition of the DNS and Bind book kicking around somewhere.

Understanding the Linux Kernel has largely been useful to me when doing anything "close to the kernel". Not a good beginner's book.

I've never used an apache book, but I enjoyed the Varnish book. This definitely helped me.

Of course, these books don't cover everything, and those listed are relevant to my interests so your mileage may vary. You'll never go wrong teaching yourself new skills though!

EDIT: I forgot about the latest book I've read. I used tmux for a little over a year before purchasing a book on it, and it has improved my use of the program.