It wasn't really a spider. Try not to let the movie influence your perception of the end of the book. The spider and the turtle were just loose representations of entities which couldn't be comprehended or expressed in words. He was pretty much channeling Lovecraft there.
It is my favorite book he's written, and the only one I really like. The rest are pretty good at best, It's the only one I'd call great.
After that, I like The Regulators and it's companion Desperation, probably his most frenetic and bizarro work.
Danse Macabre is interesting, though he's dead wrong about a lot of stuff, both factually and in my opinion.