1. there are several downloadable pdfs online.... and your library, to save $
2. you cant just read a few guides or books and KNOW how to mix... thats half of it, the other half is experience. training your ears to listen to the difference and the subtle changes... this I find the most difficult to be honest.
I really like bob katz mixing and mastering guides... but we are talking about preference here.
What I found helpful for me, was this one guys tip which was to instead of making a song a week or a month... and spending a lot of time of that one project, try making a song a day from start to finish.
I know this may sound silly... but I can pump out all the sounds in a few hours, the arrangements in another few hours, 6-8 hours later I have the material... and am ready to mix it. Do this everyday for three months... and use low volumes, take breaks.
I improve my skills very quickly due to my personality or character etc... I work hard at it and do AMPLE research though always take variations and perspectives into account, I dont hold it for truth word to word and things may vary so dont discard something if it doesnt seem right to you.
After I did this, I found a tip from this woman, she said, when you have a bunch of projects mixed and you think they are ready, take them to a master engineer, and pay for the mastering but SIT IN with the engineer when they do the song YOU CHOOSE to pay for to master.
We are not talking about the 90 songs you did in 3 months practicing ... we are talking about when you learn from that, and then from there on, produce such product as your art. I think i threw away almost all of the projects i had in those first 90 days, and they sucked.... terribly. its just takes practice for the ears, but again best advice I have is LOW VOLUMES.... the less pressure wave you disperse into your room, the more your ears will love you.