This is a free pdf from iZotope. It's written for a general audience, so you won't find any wow-I-didn't-know-that secrets, but it's a well-written introductory resource that any beginner should find helpful. Although it references Alloy and Nectar, you don't need to own any iZotope products to get something out of this document. If you already have it, note that this link is for a brand-new update to the document.
 
Download: http://downloads.izotope.com/guides/iZotope-Mixing-Guide-Principles-Tips-Techniques.pdf
 
If you don't already have it, be sure to also grab the classic iZotope Mastering Guide too: http://izotope.us6.list-manage.com/track/click?u=db5de68bfb507d9f81f2e07f1&id=3ae057b427&e=99100ff2e4
 

I often refer people to their site to read the great basic stuff they have there on mastering and especially what is going on in the various plug modules.  
 
Yep, great stuff. It's a good idea for those of us who know this stuff to go back and review it from time to time. It's always a good idea to review the basics.

Mixing guide?
 
We don't need no stinkin' mixing guide.
(or do we?)

Thanks bitflipper! Good find.

interesting find! I've read a couple revisions of their mastering guide, and I always end up getting annoyed because I do not believe it meets its objectives, and I don't believe that the majority, the VAST majority of musicians can adequately master their own recordings. That, and I think you are missing out on the opportunity to get another (objective) set of ears in on the process.

So, I admit I started reading this guide with low expectations...
 
But this is why, even when I think something will likely be a big piece of marketing fluff, I try to give it a fair shot.

This is a guide I would happily recommend to anyone. I have nits I could pick, some are purely personal approach, some are interpretations of facts, but by and large I think the guide is extremely well written, and very nearly complete. (It might be complete, the only thing I noticed missing on my first read was an explanation of expanders in the dynamics section - but I refuse to believe that anyone can write a book that covers all of mixing, so I'll be a curmudgeon and say nearly complete.)

The format is wonderful - here's what X is, here's some ways to use it, here are things to listen for in other people's mixes, and yours. Wish I'd have thought of it<G>!

Now I am a big fan of Izotope software. All of their plugins that I've had the chance to use are both useful and musically useful... not an easy feat! But recently I've become a little disillusioned with Nectar and Alloy - the approach seems superficial to me. And I still think they need to work on their marketing approach, because these are tools I would have continued to ignore had I not read the guide.

They manage to explain these new tools without coming off like a used car salesman. That's an extremely difficult balancing act, since they published the guide.

So, no matter your experience or skill level, or your experience with Izotope tools, I think everyone ought to at least skim through the guide - although I think most here will end up reading it in detail!