The High-Tech Knight by Leo Frankowski

Somehow, Conrad Stargard, faithful Roman Catholic and stalwart Socialist of the Peoples Republic of Poland, 20th Century, had been marooned in Poland, A.D. 1231.

Somehow, Conrad found himself under investigation by the Inquisition, got himself knighted, was granted his own fief, and made a few enemies.

Somehow, he had to round up a few vassals, build himself a city, and figure out how to survive armed combat against the Champion of the Teutonic Knights, one of the Toughest Men Alive.
This is the best time travel series in a realistic timeline ever written. Before these books, L. Sprague De Camp had held that title for decades with his "Lest Darkness Fall", but it was too short. Mark Twain may be the most famous with his "Connecticut Yankee in King Arthur's Court", but that is more light hearted than serious, and disappointing at the end. The Conrad series delivers on all the promise, and even after 5 books, it still leaves you wanting more. (There is a sixth book now, but Conrad is a minor character, and I've reviewed it separately.)

A reader' review
I say realistic timeline, because Conrad isn't really from our timeline. I was a little suspicious right from the start, but it wasn't until Conrad reminisced about the Mongols invading France that I thought "Hey, wait a minute". It turns out that it didn't happen to us (even without Conrad). But the historians I read agree that it would have, except the great Khan died and the Mongols had a war of succession which they never recovered from. This is often used as an example of the actions of one person changing history. I never even heard the story, until Conrad got me to look it up. Go figure.

This is an action story, with fighting and sex, where Conrad overcomes insurmountable obstacles, and usually has a good time along the way. The author doesn't just ignore the time travel though. He writes a science fiction sub-plot about that too. In fact, the author is obviously an engineer, not just because it takes an engineer for Conrad to build the things he does, but also from the way the books were planned out and crafted. Obviously the author planned the Mongol invasion and built the series around it, but he also foreshadows romantic sub-plots 3 books in advance.