I'm new to Acro (started for real this past fall), and read and suggest the following two books:
Basic Aerobatics, Geza Szurovy & Mike Goulian
Flight Unlimited, Eric Muller & Annette Carson.
I have actually been going back to these books after practicing, on a regular basis.
There is also a link on the message board "Video: 2010" that will bring you to a web site with excellent video of an instructor flying a Decathlon through primary and sportsman routines. I found these videos really helpfull.