CaLeDee: There is a book seller in the main market area (with all the stalls) - it's behind the gate that is closed when you first enter the place. The recipe seller is in the human enclave, which is behind the Inn. Instead of going down the steps to the Inn, you go up to the right, past it. I actually gave up on this quest because even after getting the traps (which also took me ages to figure out how to do), the harpies never lifted the traps to blow up the nests more than 4 times. Tried all the harpy spots and the number just stayed stuck at 4/7.
Well, thats just the gullies, if you remember, the dream cave quest also has harpies, and yes, they have nests too ;) Nearly every encounter to the blue crystal is a possibility to take a nest down. (Not the main-boss fight though)