I did all my prep for the commercial knowledge with the asa prepware book and cd (electronic practice test questions). 100 bucks or less. I currently have the cd with all the instrument rating and instructors questions too (multitasking)...
I also bought a book called '
Everything Explained for the Professional Pilot'
which is REAL good to help understand the hows and whys behind the FARs ...
let me know if you need links to any of these.
JB