There is a great resource now of quailty multitrack material here:
http://www.cambridge-mt.com/ms-mtk.htm Mike Senior's multitrack website. You can audition the tracks before downloading the multitrack sessions so you can pick what you may be interested in. The performances are very good so you are dealing with quality performances. They are all just raw wave files all the same length suitable to load up into any DAW. There are many genres too.
I have turned out to be an excellent mix engineer these days. I did not use any books. I am not sure books on mixing will necessarily help although when I started there weren't any so that made it a little harder. I learned by extensive listening to music of all genres on expensive hi fi equipment. Then copied and practiced until I got the same results. That approach worked for me. Listen, translate, engineer yourself.
Two books that I found to be very good too are 'Behind the Glass' Vols I and II. They are basically interviews with a whole lot of great engineers but they disclose a lot of very useful info. Stuff you can put into practice.
You can read a great book and pick up all sorts of things but in the end mixing is about using your ears and making the right choices with your technology so nothing quite replaces that work of just a lot of mixing using your ears.
I agree with Mike re the Yamaha Sound Reinforcement Handbook too. That is an older book these days but still very good. We still use it as a teaching resource in our sound engineering classes. Talking about sound engineering teachers I am probably one of the few here and I have that job all the time of teaching students to mix. The way I do it is:
1 Supervise the tracking and give them good advice about capturing things well at the start to make mixing easier.
2 I do a session first in the control room with everything up on a projector so they can see what I am doing. I do the mix first in front of them and explain what and how I am going about things.
3 We move to the lab where they all download the same session and they set that up on their individual computers. I still mix as well giving them instructions. Using the projector in the lab they can still see and hear what I do. I do something and then they do the same sort of thing.
4 After a while I let them loose on it themselves for a period. They are using headphones at this point.
5 They move into other areas and continue the mix on speakers for fine tuning etc..
Here is some really good advice and very few talk about this. The better quality the artist or band the much easier it is to mix. The main reason why people have difficulty with their mixes is because the music itself is simple ordinary. FACT! Sorry to say it but it is true. People should learn the art of detecting poor or average ideas and performances much earlier on and go back and create the music again until it is much better. I learned this early and these days I only record and mix stellar artists and performances. I turn down the rest. It is a waste of time trying to make something ordinary sound good.
When you hear a fantastic mix you are not hearing the mix at all. You are hearing the artist, song, band, performance. That is what is blowing you away, not the mix! In fact a great mix should almost disappear and let all that shine through.