I define 'sleaze' as using sensationalism to explore the borders of what is considered acceptable in the current literary environment. The closer a book, topic or cover gets to that line, the 'sleazier' it is. If it crosses the line, it would pick up such monikers as 'radical,' 'pornographic,' 'underground' or 'XXX.'
HOW the publisher accomplished the 'Sleaze' image sometimes made all the difference. Many of these books published in the 50's and 60's might have been marketed as hardboiled mystery or romance novels, but the publisher believed that the work could make a larger profit by packaging it as a sexually oriented work.
I sort of put 'Sleaze' into two categories. First is the type of publication that used sex to sell books that might otherwise be categorized as mystery or romance. The second type of publisher wanted to sell 'sex books' but couldn't be completely uninhibited because the U.S. Court System still had a very broad definition of 'pornography,' which was illegal. And so, they pressed the edges of that particular envelope very hard, and often had books questioned in court.
In the case of the first category (sort of the 'R-rated' sleaze books), the largest publishers were Beacon and Midwood. Beacon was the first regular paperback-sized book published by the Universal Publishing Company, which also issued such labels as Uni, Intimate, Royal Giant, Stallion and Fiesta (most of which were digest-sized books). The Beacon label would eventually give way to Softcover Library (starting somewhere around book #B800), but it kept the same lighthouse colophon.