If a book has multiple words in it like Where the Red Fern Grows you wouldn't say where the Red ferns grows

The only reason you think it works because the title of the book includes "book". So books of Mormon only works when saying lower case books of Mormon (the person) As in you want multiple books that speak of Mormon.

Book of Mormons would mean you have multiple books. But inserting an s into the middle of the title is not right.