Charlie said I would talk about self publishing. He also mentioned something about "shameless plugging". So, let me also tell you about writing a metric tonne of franchise fiction, my really very rapid writing process, and why I self published my book Storyteller Tools: Outline from vision to finished novel without losing the magic. I promise it won't be all sales pitch, except for the very last paragraph.
This time last year, I was recovering from having written four short (40K words; about the length of an old 1970s Lin Carter paperback ) franchise tie-in novels at a rate of roughly one every four months. Since each novel had to fit the setting, one of those months went on research.
These days, franchise novels, that is novels "tying in" to an existing story world, typically belonging to a video game, aren't disposable trash (if they ever were).
They're part of both the franchise's brand and that of the author, which is another way of saying that reputations ride on their quality. Not just reputations; nobody sane becomes a writer or an editor unless they like getting books right - there's no point in being (comparatively) skint and if you're not going to have fun!
Many, many, good writers mix franchise with original fiction. Two examples you might have missed are: Paul S Kemp, Star Wars fablemeister who also writes the exquisite Egil & Nix books (which I will get to in part 3 of my Defence posts); and Howard Andrew Jones who writes Pathfinder adventures, but is responsible for the amazing Desert of Souls, basically Arabian Nights fan fiction and a good example of non-western Fantasy.
There are good reasons for writing for hire.