If you can get your hands on it, Sound Design in the Theatre by John Bracewell is one of the best theatre sound books there is. It's officially out of print, but now that John has the rights back from the publisher, he prints copies out at Kinkos and sells them to the Ithaca College Bookstore for basically the cost of the printing. If you want to look for the published 1993 edition, it's ISBN 0-13-825167-3.
It's got everything from detailed explanations of sound system components to electrical theory to human hearing to psychoacoustics. And it even has a whole part devoted to the sound design process.
What areas of sound reinforcement are you interested in; system design, system operation, production, etc.? Do you want to stay on a practicallevel or get deep into the science and engineering behind it?
The books are for a paper i have to write to graduate from my highschool. My topic is basically how sound is done for plays, musicals and prolly concerts. Like how the setup can be different for a musical than a play. Things like that.
Not to promote another web site, but if you go to www.prosoundweb.com and look at the study hall section, and the forums you will be able to get a vast amount of information
The Yamahabook is an excellent starting place, but IMO for far more indepth info check out theprosoundweb site