Lords and Ladies by Terry Pratchett.
The witches Granny Weatherwax, Nanny Ogg and Magrat Garlick return home to the little mountain kingdom of Lancre, where they discover that amateur practitioners of witchcraft have weakened the fabric of fantasy. Elves manage to break through, and must be defeated.
The plot turns on Magrat's wedding to King Verence II, and her misgivings about it. The plot includes several elements of A Midsummer Night's Dream by William Shakespeare, and quotes lines from various folk songs about elves. The sub-plot includes the Unseen University wizards, mainly Archchancellor Mustrum Ridcully, Ponder Stibbons, the Bursar and the Librarian.
According to Pratchett himself, this is one of the few Discworld novels that cannot stand on its own; one needs to have read the previous two books in the Witches sub-series, Wyrd Sisters and Witches Abroad, to have a proper understanding of the characters and their relation to each other.
