Out of all the books on mixing out there Mixing Secrets by Mike Senior gets our vote for the best one. If you are mixing your own music this book is essential reading that will no doubt improve your music if you take the time to practice what Senior says. He talks about everything you need to know in order to achieve a good mix and gives you step by step examples. He talks in plain English and is easy enough for anyone to follow.  It also contains tips that even more experienced engineers will benefit from. Great book for the DIY musician.