I stumbled across this website a couple of years ago, 2012ish, when my daughter remarked that a particular series about cats (ack! sorry, had a hairball there) could not possibly be written by the same person, there were too many of them (books) and they all felt...different, from book to book.
This agency has a standing application for authors to apply to work for them. I don't know the terms, but you generally get paid a flat fee for writing like this, and you don't get to say you wrote "xyz" until much later, but it can depend on the contract your agent negotiates, I guess. I've never done this, so I don't really know. We all know it's done, I had just never seen an agency advertise for ghost writers like this.
It's an intriguing idea, and a way to learn the craft by...well, writing, although you've gotta be pretty good to start. They provide the well-plotted shell, with already established characters, and you flesh it out. They read first chapters from several applicants, then choose the one -- the writer -- they feel does the best for the series, and bango! You're writing a book, and learning what a best-selling series writes like. It's gotta be a great education!
Some authors my daughter really likes got their start here, for example, for that (ack! -- another hairball) series I mentioned earlier (and is still one of her favorites - down to reciting names, clans, etc.) and a dragon series writer who so generously signed all her hard-cover books at the last Tucson Festival of Books. We absolutely adored the Dinosaur Cove series, when my kiddos were much younger and in early chapter books.
In any case, here it is. Working Partners.