The drop rate if inversily proportional to how much you want it.
I know when I was lookign for it I weent hunting there for a few days. Finally gave up. A clanny of mine say one in town and bought it for me. A few weeks latter, I go back to EV to hunt there for xp. Guess what, got 3 books in 1 hour.
My suggestion: Go hunt in EV with buddies around the same level to try to get the SB. While your hunting make a few shouts asking if anyone has it. Normally if someone is hunting for another SB, and they just happened to have purify drop, they will normally just give it to you.
There are a couple of books that are some what hard to get and in high demand in EV. Purify is one, guidance, and death whisper. But hunting for it while shouting is a great idea, that's what I did.
I seem to recall that the Purify SB drops from the Forest Runner in EV. They aren't everywhere, just certain areas in the upper part, and maybe one very small section in the lower part. Keep looking till you find an area with them in upper EV, spend a while grinding, and you'll get it.
Go to EV South. Run along the lower level until you get to the point where you run up the slope on the left to the upper level.
At the top go left and head to the bridge (over the little hill and to the right a bit.)
Over the bridge turn right and run up the little slope. This leads to the big open area where there are the mix of Elders (Unicorn, The floaty women things, Tree and the Satryr). Run across this bit on a digonal to the opposite bridge. Run across that and you will find the Forest Runners as OldFart said.
If you are not high enough level to fight in upper EV, take someone with you as most of the mobs aggro and they can hurt.
There are other places but that's where I went to and it's easy to find.