I couldn't disagree more about the quality of Pyramids - a standalone book that encapsulates so many ideas and humor - TP's minds must have been quite literally buzzing with thoughts that he tried to cram in - my favorite books are the Guards sequence but I loved this one (to the extent that I have it on audio cd in my car) and it was also the book that totally got Mrs. Randywine into the whole discworld universe (probably due to her love of all things egypt-ish).
I think also that over several books too much has been written about the assasins school to allow a novel to be largely based there, there would be considerable duplication and redundancy.

Cheers

R.