Showing Night Train to Memphis by Elizabeth Peters

Vicky Bliss is the first to admit she doesn’t know a thing about Egyptology. But her familiarity with criminality brings an intelligence agency to her office with an offer she can’t refuse: they want her as an undercover operative on a luxury Nile cruise because certain information has come their way that a major theft of Egyptian antiquities is in the works.

Vicky suspects the man they are seeking is her occasional lover and frequent adversary, Sir John Smythe.Then, on the first day of her Nile cruise, she spots him – with a beautiful woman clinging to his arm.

Stunned and furious, Vicky is too preoccupied with her own feelings to concentrate on crime on the cruise – but then one of the crew is brutally murdered and Vicky finds she must put all her emotions aside and join forces with her duplicitous lover if she wants to solve the case…