Still Waters by Nigel McCrery

DCI Mark Lapslie is on medical leave from the Essex Police Force when he's called back to work on a case. As the result of a car accident, a body has been unearthed with injuries that bear a resemblance to those seen in a previous case that Lapslie worked on. Lapslie has a condition called synaesthesia that causes him to taste sounds. It has gotten so severe that he had to separate from his wife and children. Now, called back to lead a murder investigation, Lapslie tries to keep his synaesthesia in check while working with DS Emma Bradbury. For some reason, someone is following their investigation very closely and Lapslie has been told by his superior not to spend too much time of the case. Despite these odds, Lapslie is on the trail of a killer who preys upon and poisons older women. Will he be able to catch her before she strikes again?