The Directory of Open Access Books (DOAB) is considered to be an important registry for quality-checked Open Access books – even if not all peer reviewed books are listed there. Two days ago, on 23.01.2018, I took a quick look at the ratio of books per publisher.
On that day, DOAB listed 254 publishers who had registered 10,502 books. The arithmetic mean was 41.35 books per publisher, the median was 10. Accordingly, the majority of publishers publish very few books. In fact, 92 publishers (36% of all listed publishers) reported five or less books.
On the other hand, 19 publishers (7,5% of all listed publishers) publish 5,373 books and thus more than half of the books reported (51%).
The extremely trivial data can be downloaded here as an OpenDocument file.