Maskerade - Terry Pratchett Nanny Ogg manages to write a best selling cooking book, but after Granny Weatherwax discovers that the publisher has stiffed Ogg out of a load of royalties they head off to Ankh-Morpork to set things right.

Agnes Nitt, tired of being the girl with a nice personality has also headed off there to join the opera (rather than getting roped in to being third witch).

All is not as it appears in the opera, with a mysterious ghost terrorising the staff.

The witches get pulled into all of this as they go to check up on Agnes, and start using their own special way with people to sort it out.

The usual mix of good characters, humorous situations and quite a few laugh out loud moments.
Pratchett's witches books always seem to be some of the better ones in the Discworld series.