Agatha Raisin and the Witch of Wyckhadden by M.C. Beaton
Left with bald patches thanks to the wicked doings of a murderer from a previous investigation, Agatha flees to the coastal resort of Wyckhadden to grow her tresses back in privacy. When a local witch provides her with hair tonic Agatha's hair begins to flow - but the witch is found bludgeoned to death. The odd elderly residents who inhabit Agatha's elegantly faded hotel seem innocuous enough, but as she delves into the killing she turns up secrets best left undiscovered, and powerful motives for revenge. Balancing the amorous attentions of police inspector Jimmy Jessop with an increasingly treacherous search for the culprit, Agatha is at her wit's end - and ready to cast a spell of her own...