Mail order sources - call for catalogs or quotes:

Yahama Sound Reinforcement Handbook, Second Edition, Davis & Jones, Hal Leonard Publishing, 1989, 417 pages, available from Music Emporium or Mix. Much more detail on many of the items in this booklet. Highly recommended.

Electronic Musician magazine has some articles on practical sound system work. I've found it useful, although it's oriented to us closet synth/computer freaks.

Mix Magazine is the next step up in sophistication. You have to be pretty heavy into it before this will make much sense. The Mix Bookshelf, however, is their collection of books on related subjects, many of which start at a basic level.

Don't forget to listen a lot and carefully to your favorite traditional music, especially if you're not a player yourself. This will help familiarize you with the sounds and roles of the different instruments. It will also show you how your favorite bands thought they should be mixed. I've learned a lot this way.