
I couldn't tell. Kept could be by either one as the prose is smooth and very readable, just as both Westlake and Block can deliver. I read the book in Finnish translation (it was published as Maksettu rakastaja in the mid-sixties by Finnbooks in their short-lived series called Domino) and for all I know, it could be abridged or altered in any other way.

I was rather disappointed in Kept, because I went in looking for a criminous content, but there was none! This could've been a romance paperback, save for the fact that there are some candid sex scenes (candid for their own time, mind you) and that the lead character is a man. The book starts off promisingly, a bit like Postman Always Rings Twice, with a beautiful, young woman picking up a hobo man off the side of the road. But in the end nothing much happens: the boy gets the girl and that's about it. There's some fascinating Mad Men territory being covered here, though: penthouse luxury, new hi-fi stuff, well-cut suits, bossing women around at the office, drinks consumed almost at all times, all that.
But all in all, I'm not sure if I share the Vintage Sleaze Paperbacks conviction this should be reprinted. (But take this with a grain of salt, since I read the translation.)
The great original cover is by Paul Rader.
By the way, here's a link to the Finnish Domino series. Any comments on the books published in it?