I concur with the post above - Roey Izhakis "Mixing Audio" is my best buy ever, and if you buy one book only, make sure that is the one.
That said, Mike Senior's book "Mixing Secrets" is my third favorite, only topped by the cult classic "The Art of Mixing" by Dave Gibson (who is absolutely brilliant in presentation).
Make sure to see his accompanying video if you can get it.
Apart from those three you could look up "The mixing engineers handbook" by Bobby Owsinski as well - even though this is not as in depth as the others and half the book is interviews
with people in the business.