I'm looking for a book I read in the early 1990s that is probably a bit older than that. I picked it up in paperback off a used-book rack in bad condition, and it had one of those extra-generic 1980s painted covers, so might have been a decade older. For some reason, when it came back to my mind, I thought it was by David Drake, but nothing I can find from his limited fantasy output matches it.
The basic worldbuilding conceit of the novel is that everyone has a D&D-style "class" and experience level visible to everyone as glyphs or tattoos on their forehead, which apparently appear by magical means; they aren't applied by the people themselves. The glyphs are easy to interpret, for example sailors have anchors on their foreheads, and more anchors means a better sailor. Likewise, merchants all have barrels.
Another interesting detail is that the world is relatively small and organized around a circular river. It's not a canal; there is a current and so forth. However, sailing downriver for long enough will eventually loop around to the same places that were passed upriver.
The plot revolves around a vague conspiracy, which I recall as just "basic economic literacy" without remembering the details. The key point is that it's carried out by a certain character class with distinctive forehead markings that no-one can recognize or understand. Apparently all people with this marking are involved in the conspiracy, so it amounts to an indication that the person cannot be trusted; as a result, they must be very indirect.
The main character is recruited to investigate the conspiracy by some members of the merchant class, because he is a Conan-type superior physical specimen. This is indicated not only by his physique, but because he has a large number of swords on his forehead, more than anyone has ever seen before. However, it turns out that this isn't the whole story; his body is that of a great warrior, but it has had someone else's mind stuffed into it. This is a reader-relatable character who "died in the real world" and then found his real-world personality transferred into the impressive body.
So far as I'm aware it was a standalone book, not a part of a series, despite setting up quite a lot of worldbuilding tropes as I've mentioned.