Small Gods is arguably the best book he ever wrote! Very funny a very deep at the same time...
Another real good one is Guards Guards! (a secret society summons a dragon, spoof on The Hobbit and secret societies, one of the members of this secret society is Brother Watchtower... :) )
Good Omens written by Terry Pratchett and Neil Gaiman takes a spin on Revelation and end-time prophesies, witnesses are mentioned ofcourse. Very Very Funny!!
Interested in science? Read The Science of Discworld. This is not like those other books with The Science of in the title like The Science of Startrek. Rather, it is a humorous look on the origins and headings of science, very deep. It uses the wizards of Unseen University to illustrate parts of current science and to add some humor to it all.
I enjoyed reading Pyramids but do not consider it very good. I liked the inside look into the assassin's guild, yet the rest of the book was...well, not that good.
Well just my two cents...
Greven a big fan of TP, Douglas Adams, Black Adder, Monty Python etc etc