Agatha Raisin and the Murderous Marriage by M.C. Beaton

Agatha Raisin and the Murderous Marriage (Agatha Raisin Mysteries, No. 5) - Agatha is finally getting her wish, she is going to marry her neighbor, James Lacey.  However, her 1st husband, Jimmy Raisin, who she hoped was dead, shows up at the ceremony announcing that they were still married.  That evening, Jimmy is found dead and Agatha and Lacey are both suspects.