"Mixing, Mastering and Production" by Waves (the plug-in people) is excellent. Its about £50 but extremely useful. I dunno about everything they say but on the whole an interesting take on the whole thing. It helps if ur a little bit down the productional path so u can put the knowledge into context, but still good for intermediates. Covers mixing and mastering a trance tune, a rap tune, a rock tune and a country tune (I think).

Its a big book (approx 200 pages) and comes with 5 cds, including the finished tunes and screenshots of EQ, comp, fx, etc. Basically walks u through the mixdown process explaining what was used and why.