I agree with Les's suggestion of the
Yamaha Sound Reinforcement Handbook, there is no other
book that goes as in depth with detail as it does. Though, having read it, it is a very dry and technical
book that can be very difficult to get through and understand without a strong basic knowledge of sound theory and sound equipment.
I would suggest that you first take a look at several other books such as:
The Ultimate Live Sound Operator's Handbook by Bill Gibson
or/and
Live Sound Reinforcement by Scott Hunter Stark.
I agree with the
Yamaha Sound Reinforcement Handbook but have to strongly disagree with the Gibson
book. There is some good information presented in The Ultimate Live Sound Operator's Handbook but that is mixed in with a large quantity of misinformation, myths and misconceptions. Much of the same content, but with a different introductory portion, is found in Gibson's The Ultimate Church Sound Operator's Handbook. Both books are easy to read and thus attractive to less technically experienced readers, however being easier to read does not justify the misinformation presented and unfortunately, the less technically experienced readers are the one most likely to be unable to separate the good info from the bad. Personally, I get the perception both in the content of the books and in the responses of the author to criticism of them that these were written by someone who is more concerned with appearing to be an expert on the subject than with actually being one.
I've personally not had a chance to read the Stark and Fry books suggested but have heard good things about them from others. The JBL Audio Engineering for Sound Reinforcement
book by Eargle and Foreman is greatly a compendium of information from other sources, but also seems to be well received for being easier to read.
All the books suggested so far seem to focus on the technical aspects of mixing and live sound, but don't touch much on the process of sound design or
acoustics and psychoacoustics, all of which can be important elements in creating a desired sound environment. For
acoustics I would suggest the Master Handbook of
Acoustics by F. Alton Everest. And for a more
theatre focused option, Sound and Music for the
Theatre by Deena Kaye and James Lebrecht addresses much more the process and
role of sound design than the technical aspects.
I'm a
bit surprised at the references to the
Yamaha Sound Reinforcement Handbook being overly technical. It is much more a textbook than a narrative but for
system designers and audio pros in general it is widely considered the comprehensive introductory text that sets the
stage for more advanced works such as Sound
System Engineering by Davis and Patronis, Sound
System Design and Optimization by McCarthy, Audio Systems Design and Installation by Giddings, Handbook for Sound Engineers by Ballou,
etc., all of which are more technically advanced texts common among audio pros.
The Sound Reinforcement Handbook is a little dated, as are some of the others noted, but talking to the authors of some technical books, one of the challenges faced is that the publishers may only want to issue new editions of technical books if there is sufficient new content that it will significantly increase sales. At the same time, the publisher sometimes owns at least some rights to the original content, meaning the author can't go elsewhere to publish unless they start all over with a completely new
book. I'm not sure if this applies to the Sound Reinforcement Handbook but I know it is what has prevented some other audio related books from having revised versions issued. A few months ago Don Davis also mentioned that he and 'Doc' Patronis had signed to create a Fourth Edition of Sound
System Engineering, so there will be at least one more edition of that seminal
book.
In general, the issues I, and others, have had are not with the mixing techniques aspects but rather with the more technical aspects. For example, the roles of contractors, dealers, salespeople, Electricians, consultants, 
