Old smut paperbacks have been seeing quite of a renaissance in the last couple years: there have been lots of blog posts about them, there are Facebook groups for them and associated stuff (try this) and what's the most important thing, there have been quite a few reprints. One of the most interesting of them has been the collection of Robert Silverberg's two sleaze paperbacks, called Gang Girl/Sex Bum. It's a very good book which I recommend highly, if you're into noirish and nasty little crime stories.
Gang Girl was first published as a Nightstand book in 1959 and it was one of the first of Silverberg's many sleaze novels. It's a juvie book, about a 16-year old (!) girl trying to break into a new gang and making her way up to the top. Some of the sex scenes are downright nasty, especially the gang rape scene that goes on and on. The ending is very noiry and fits the bill. Sex Bum, from four years later, is a better book in my opinion, though Gang Girl is not bad in any sense of the word. Sex Bum, this time a Bedtime book, tells about a young guy living in a hicksville wanting bad to make it into the mob. There are lots of sex scenes, but coupled with the crupulousness of Johnny Price they make a chilling read - this guy feels like he's stepped out from the pages of Jim Thompson of Jason Starr. The ending is very chilling.
These books were written fast to make fast money (and money Silverberg made: he bought a big house in Greenwich Village with the money he got writing these books!), and it shows somewhat. There are some meaningless characters in Sex Bum, and Gang Girl suffers somewhat from being too episodic, but I don't mind, as both books are also immensely readable. As there's also a foreword by Silverberg himself and an afterword by Michael Hemmingson, plus a 3-page bibliography of Silverberg's smut books, this books comes very highly recommended. Can't wait for this to arrive.
More stuff about Silverberg's sleaze books here.