I recommend the Yamaha Sound Reinforcement Handbook, I think everyone dealing with audio shit should have it, you probably know most of what it contains, but it's a great reference.
[ame]http://www.amazon.com/Sound-Reinforcement-Handbook-Gary-Davis/dp/0881889008[/ame]
That Modern Music Mixing Engineer or whatever book that I see recommended everywhere seems kind of lame. I paged through at a bookstore and didn't find anything beyond basic knowledge.
I will say, buying books for the specific software/tools you use can sometimes be a great investment. Seeing someone else's workflow and learning features you weren't totally aware of can help you become a more efficient user.
Music Law: How To Run Your Band's Business is ok primer into legal bullshit.
I also always look for interesting books on business when I'm at thrift shops, the business side is not as interesting, but it's very useful.
I know I just zapped a bunch of not online/free books, but that's off the top of my head. I'll see if I have any useful online info.
