Sharp-eyed reader JMW has let me know of another duplicate use of stock on book covers: it's a photo gracing both Poker Face and The Ladies' Lending Library.
As he says in the comments to this post: "What's interesting is that, on the Lederer cover, the woman is holding an ace of spades in her hand, showing it to the camera. In the Keefer photo, she's not. My first notion was that the ace was placed in for the Lederer cover, since it seems like an odd thing for a woman on the beach to be holding. But on the Keefer cover, the way she's holding her hand seems strange for someone not gripping anything. Huh." Also, she's had a bow added to or removed from her hair, and had her swimming costume altered to show either more or less thigh.
Also weirdly, the Poker Face jacket credits the photo to Donna Day, while the Library credits it to Lyn Szynkowski.
A bit of hunting around (no, I don't have anything better to do, actually), finds us the original: Donna Day is the photographer, and the original model is holding a camera. So now we can all get some rest.