There Goes the Bride by M.C. Beaton

Agatha Raisin's ex-husband, James Lacey, is getting married to a much younger woman, Felicity Bross-Tilkington. Suddenly, Felicity is shot to death just before the wedding. Agatha is considered a suspect because she was overheard telling James to shoot Felicity to get out of marrying her. Still, Agatha is hired by Olivia, Felicity's mother, to find out who murdered her daughter. Agatha, along with the other detectives from her agency and friends like Sir Charles Fraith, attempt to find the killer as more bodies pile up. Agatha also is still trying to find love and wonders if her best detecting days are behind her. I so love this cozy mystery series even after twenty books.