Its very very rare. I was going to do 2 words of darkness. stopped at 1. No way in hell is it 31% chance to drop. Although Icy Goblin is kind of common, probably 1 in 4 or 5 spawns.

go down ramp into jail area, take a left, then its the first door on right, middle goblin.

Thanks for the info. Maybe I got lucky, but I killed about 6 PHs and 4 Icy's and got my book.

Derp, nevermind.