When I started in the business, there was a thing called adult fantasy, but nobody quite knew what it was, and most publishers didn't have an adult fantasy list. They had science fiction lists, which they stuck a little bit of fantasy into. ↗
Books on horse racing subjects have never done well, and I am told that publishers had come to think of them as the literary version of box office poison. ↗
As for collaboration - I have done a lot, 26 books, and found publishers increasingly resistive to them. It's not that the books are bad; editors won't even read them. ↗
I do one Xanth novel a year, because at the moment that is all that publishers will accept; they don't want any other type of fiction from me, so Xanth pays my way. ↗
I had some connections from the newspapers that I did work with up there, so there was a newspaper publisher in Hollywood, and they promised me work and so on. ↗