I write fanfic - and also write mysteries. Most of them have been short stories, but one of those short stories blew up in my face and turned into a novel. A small publisher picked it up; the books came from the printer two days ago. Right now, it's only available on the FTL Publications site - it takes a bit longer for small publishers to get on Amazon. Maybe by mid-October.
The book is called Secret Murder: Who Shall Judge?. Secret Murder is a key concept in Norse law, back during the days of the Vikings. Killing people was okay, but you had to own up to your deeds and be judged by the community. If you didn't announce your deed, you were guilty of the much more serious crime of secret murder. If the community judged the victim needed killing, you were pretty much home free, though you probably had to pay compensation to his relatives. If the community judged you as being in the wrong, you could be outlawed. That meant you were no longer under the protection of the law, and had no legal complaint if somebody killed you. Outlaws usually moved elsewhere in a great hurry.
In the lands of the English, things were decided differently. The King and his nobles did the judging, and usually confiscated the goods of the killer.
Things get difficult when the outlaw moves to English lands, and becomes a prosperous (if unethical) merchant. When Norse traders come to town, their old, outlawed, enemy turns up dead. Who will catch the killer, and whose law will he be judged under?
If you want a taste of the world of Secret Murder, I have a related short story at Thorolf and the Peacock.