Oh hey i stumbled upon this! I'm a great admirer of Bill Watterson's work too!
What bummed me was albeit there are many books, fair part of it are repeats hence I didn't buy all the books, I tried buying those that didn't have too much of repeats.

I have to double check my library, off the top of my head it would be

Authoritative
10th Anniversary
Essential
Revenge of the babysat
SOmething about things going boink at night (cant remember)