There is actually a website named Plagiarism Today. It doesn't lack for content.
Also, there is now a posted step-by-step response protocol for when you discover that your work has been stolen.
I've been a "ghost writer", although never for fiction. I would clean up quickly published books written by politicians just prior to their attempts to gain their party's nomination for whatever office interested them or athletes upon their retirement or bored captains of industry. Frankly, I've never heard of ghost writers for fiction. That sounds kind of ewwww. Fiction has always been such a personal genre that I can't imagine any author giving up what seems like their child to some stranger. Let alone one from a ghost writer service.
I recall sitting as one of the clergy at the ordination of a priest when, while listening to the sermon, I realized that the preacher had stolen about one-third of one of my ordination sermons. It was one of my first sermons to be published online and also, for that reason, one of the last.
On another occasion, when I was a new rector in a parish, one of the couples from the congregation brought their friend's adult education curriculum to me so that I might use it at our parish. Since he was a Congregationalist, I was curious to see what he had developed, since most of the Congregational clergy I know merely assimilate from other sources.
Turns out I was more right than I thought. What they handed me was a direct copy of the curriculum of one the courses I had designed and taught at Hartford Seminary. It was online, you see, and all the congo fellow did was remove my name and replace it with his.
So, I understand how this could infuriate an author.