There's a stigma when it comes to writing erotic books making it a whole lot more complicated than other genres. Generally speaking erotic books revolve around the simple premise of characters performing a sex act with the rest of the plot as filler leading up to it. Sometimes the subplot is also erotic or another genre entirely such as erotic romance, erotic steampunk, erotic comedy, etc.
I have noticed however there's a tenancy to feel awkward or embarrassed about writing such stories that doesn't apply to other genres such as horror for instance which may be incredibly violent and disturbing. In some ways writing about a werewolf ripping out someone's guts in a gory frenzy is considered fairly normal whereas a story about two lovers giving each other anal sex could be seen as perverse. I could just be me, maybe I'm a prude or easily embarrassed. I suppose it's down to the reader making assumptions about the author's inner thoughts. If someone knew I thought about werewolves killing people they'd know it's just a fantasy whereas if a reader knew of my sexual thought's they'd wonder if I actually partook in such activities.
Most of my stories tend to be set around mythology, warriors and fairies but it's erotica that sells. It's a sad fact but when it comes to publishing ebooks a ten page erotic short will far outsell a three-hundred page novel, yet on the other hand it's far less likely to receive a review. My guess is people don't like to admit they read erotic books so avoid reviewing their purchase.
"The difference between pornography and erotica is lighting."
- Gloria Leonard.