Some of the best-selling in terms of what genres make the most money genres include romance/erotic fiction, mystery fiction, science fiction/fantasy, and religious/inspirational books. Romance/erotica fiction is the one which makes the most revenue probably because the fact that, for good or ill, it is easy to sell something if it is somehow attached to sex.
A close second is mystery fiction, particularly murder mysteries. There is something fascinating to many people about investigating a murder and unraveling the secrets and the intentions of the killer. Another genre that sells well is science fiction and fantasy probably because of interesting ideas as well as the sense of adventure and wonder that surrounds that genre. Another well selling genre is non-fiction books on religious or inspirational topics.
The Bible, for example, is the best-selling book of all times. It also reflects the popularity of self-help and inspirational literature. These are the genres that make the most money though there are other ways to measure the popularity of books such as reader’s preference and what publishers normally most want to publish.