So many book reviews elsewhere on the net, so here are some of the good one:
Wicked Cool Java - Programmers and technical leads will love this book
Building Online Communities with Drupal, phpBB, and WordPress - Three popular web PHP based open source applications, it is fun to read and enhance your knowledge.
The ABCs of LDAP - a good book on LDAP
Postfix: The Definitive Guide - Anyone who works with Postfix should really read this book
Beginning Python: From Novice to Professional - Tour the Python language, from basics to advanced modules.