Get yourself a copy of the FAA test prep either in Hardcopy or cd-rom or download from the net.
If you plan on taking the course then get this out of the way first.
Alternatively, you can study this required aspect of the programme on Sheffield's website.
If you have no prior aviation experience then you might want to check out some of the weather books that can be bought on amazon and sportys.
There are no dispatch specific books on the market, least none that I've seen and most of the info you'll need can really only be gotten by using the existing pilot training texts.
As I say, the FAA test prep is a good one to start with as you need to take this test anyway to get the licence and it's better to have it done before you go to your dispatch school. It's self study too so you can do it at home in your own time.
Hope it helped
Hustle on..
