I read these book(s) back in the 70s in SF Book club editions, I think. Possibly a trilogy? At least one of the books might have had ‘Engineer’ in the title.
The plot was basically “A Connecticut Yankee in King Arthur’s Court”, only the hero goes back to medieval Poland, gets befriended by the king, and proceeds to introduce engineering breakthroughs and so Poland turns into the heart of science/tech/industrial innovation. (I suspect the author was of Polish ancestry and was avenging too many years of ‘backward Poles’ jokes.)
A random bit that stuck in my mind: Somehow as a result it becomes normal for maps to be shown in an orientation other than North at the top.
The plot also sounds a little like that of Three Hearts and Three Lions, by Poul Anderson (an excellent book, BTW), though as the protagonist in that one is a Danish engineer, I’m pretty sure that’s not it (and, it also looks like @What_Exit has a better answer).
I think I read the first couple of books and the story seemed to be getting progressively more self-indulgent (e.g. less about industrialization and more about a schlubby middle-aged guy having sex with hot women). I much preferred de Camp’s “Lest Darkness Fall”.
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Do you have a reference to the book? I tried to search on the title at kobo.com but get several books that at first sight might qualify (time travel, ‘the traveler’).