The 23d book in Terry Pratchett's Discworld series. It features the Witches (not the Wizards or the Watch) and their fight against the vampyres descending from Uberwald onto the friendly little kingdom of Lancre.
Not as funny as many other books in the Discworld series, a bit eerie at times (maybe I just don't like the subject matter much) it has its funny moments - often - as with Pratchett just a single word in a unexpected context (clustersuck).
Necessary reading for rabid fans of the Discworld, others should probably start somewhere else (Witches Abroad being my favorite Witches book).