KILLERS OF A CERTAIN AGE by Deanna Raybourn:
What a great plot set-up: Four female assassins just turned sixty and have been forcibly retired. Their employer sends them off on a Caribbean cruise to reward them for all those years of loyal service. They discover the employer does not intend for them to survive their vacation. An action-packed thriller told in Deanna’s terrific voice. Oh, and you will learn stuff because Deanna’s research is impeccable.