Just finished Maskerade.
I have to say, this has entered my Top 5 Discworld books I think and is definitely the sharpest 'witches' book I've read so far. Everything works; Granny and Nanny are great (particularly in their 'roles'), Agnes Nitt is a really likeable new character, Walter Plinge, Henry Slug, Salzella and their alter egos are great fun, you've got some of the Watch in there, the Librarian makes an appearance...it just all works.
It's a fusion of everything - kind of like Wagnerian Opera I guess. With the number of characters in this book from different story arcs and the witches in unfamiliar territory I suppose there was a danger this could have just become a collage, but it's a good, tight story.
Carpe Jugulum up next...