Eye opening book
I think I've read a good number of the Books on Google
but yesterday I read Google Hacking for Penetration Testers
. It's written by hackers who've come up with incredibly sophisticated google queries that can find passwords, intranets, credit cards, logins, system files, etc. All stuff that could be used to hack systems, or used by security admins to find vulnerabilities. It's an amazing work of genius really.
If you want to take advantage of the techniques, you can download wikto, which can use the google API (the original search API) and a database of queries created by Johnny Long (johnny.ihackstuff.com)