Agatha Raisin and the Witch of Wyckhadden by M.C. Beaton

Agatha Raisin and the Witch of Wyckhadden (Agatha Raisin Mysteries, No. 9) - After a vengeful hairdresser had shampooed Agatha's hair with a depilatory, Agatha has run to Wyckhadden until her her grows back.  She is staying in a hotel that has a few elderly permenant residents.  One of them suggest she sees the local witch to obtain a potion for hair growth.  Afterwards, the witch is murdered and, of course, Agatha is one of the suspects.