The Wee Free Men

The Wee Free Men is the 30th novel in Terry Pratchett's Discworld series, published in 2003.

Warning: Wikipedia contains spoilers

The Queen of the elves (see Discworld 14, Lords and Ladies) has another try at invading the Discworld, stealing kids and infesting people's dreams.

With the help of the Wee Free Men, the Nac Mac Feegle (see Discworld 23, Carpe Jugulum), 9 year old Tiffany finds out that her grandmother used to be a witch, and that she has inherited the trade from her. So when her baby brother is stolen, Tiffany and the Nac Mac Feegle enter the elves' world to steal him back.