Gay erotica/romance with BDSM elements. Often very politically incorrect, depicting slavery, nonconsensual sex and even rape. Yes, I own quite a lot of the genre and love some of them. Examples:
- Bloodraven by P. L. Nunn
- Tribute by Lisa Henry
- Dark Heart by Thom Lane
- Prisoner by Megan Derr
- The Administration series by Manna Francis (not erotica, but pretty good science fiction. The two male protagonists have a BDSM relationship, though)
I know I said books with naked men on their covers are out, but this rule doesn't apply when I'm looking specifically for erotica. Very seldom nowadays, but I have a sizeable collection from my "erotica period", about 15 years ago or so.
Then many fantasy books. LOTR, The Wheel of Time, the first three ASOIAF books and several others.
An Exchange of Hostages by Susan R. Matthews (science fiction)
Several classics. King Solomon's Mines and the Mowgli stories (supposedly racist), Jane Eyre, Pride and Prejudice (romances), there are others.