Killers of a Certain Age by Deanna Raybourn [in Booklist]
Billie was the outlier in 1979 when she was recruited by the Museum to become one of four elite Sphinxes: hired assassins who do good. Now as newly retired sexagenarians, she, Helen, Natalie, and Mary Alice board a luxury cruise, a thank-you-for-your-service gift that turns...