#1
Hey, I'm just wondering if you guys know of any good literature on the topic of musical improvisation. I'm looking for intermediate-advanced improv or jazz improv, not complete beginner 12 bar blues BS, haha. Thanks in advance.
#3
I could find 5000 books on google on the topic, I'm wondering which are actually useful from other's experience. Hey, thanks for nothing though man!
#4
Well, I don't know any books, but you could try amazon.com.
That being said, Guitar World (and maybe Total Guitar) will have an article on improv every now and then. Guitar World (if I remember correctly) also has a jazz article every month, and I'm sure you could apply any technique you learn to improv without much of a problem.

From my experience, improv comes down to practice. Lots of practice.