Before court cases in 1967 opened the doors for more explicit titles, there were a number of publishers who published soft-core titles. Using non-explicit language and innuendo, these books were very tame by today's standards. In many cases, the most suggestive part of the book was the cover.
These are now prized primarily because of their cover art, or because authors who subsequently became famous put food on their tables by writing these books, many under pseudonyms. This includes authors like Robert Silverberg, Donald Westlake, and Lawrence Block. Some of Block's titles from this era are now being released as mysteries.