Alibi for Isabel — Mary Roberts Rinehart

Isabel Eaton was intending to murder her horrible husband--and she didn't plan on getting caught. So she needed an airtight alibi. She knew just the person to supply it: the highly respectable and respected socialite Sally Fielding. Isabel hadn't seen her old friend for years, but Sally could hardly refuse. After all, she owed her this little favor--because it was Isabel who had covered up for Sally when she had murdered her own husband...