Unfortunately there is no simple way to answer that question – to paraphrase Supreme Court Justice Potter Stewart: I know it when I see it.
Typically a good rule of thumb is to think of other mediums, such as television or movies. If your book would get an R rating, but would fall short of the NC-17 classification, you’re probably safe to mark it as not explicit.
*Please note that our partners do reserve the right to list a title as explicit should they feel it applicable.