The Cross-Time Engineer

Series of books written by Leo Frankowski in the nineties. The plot involves a Polish Engineer from the late 20th century who is thrown into the mid-thirteenth century because he gets into his cousin's time machine while completely hammered. And the protagonist knows he only has ten years to turn Medieval European Poland into an industrial superpower , or else the Mongols will completely overrun the country and murder everyone.

These books are known for being extremely technically written, and the author as much as admitted that mainly Engineers and military buffs would like his works. In addition, the author has written "others, mostly Feminists, Liberals, and Homosexuals, seem to instinctively hate my guts", so beware of that if you plan to read these books.

This series provides examples of:

Alternative Number System: The new civilization Conrad starts up in Medieval Europe uses base 12 mathematics, as it is more convenient for division (being a multiple of 2, 3, and 4).
Beastly Bloodsports: One of Conrad Stargard's first controversial acts is to euthanize a bear that's being used for bear-baiting.
Creator Provincialism: Poland features very prominently in the series.
The Engineer: Conrad.
Fish Out of Temporal Water: The entire premise of the book series.
Mundane Luxury: Father Ignacy is able to prove to Duke Henryk that Conrad is from the future simply by showing him several pieces of toilet paper Conrad had given him.

Duke Henryk: People who can afford parchment to wipe their butts are richer than anyone in this century!

Our Time Travel Is Different: Conrad's cousin can send stuff back in time to help him, but it only goes one-way and thus, Conrad can't simply go back to the future.
- This is because he was discovered only after having been stranded for ten years, so that event was set in stone. Nothing was known about his financial state, so it was justifiable to drop a chest of gold and silver in his path.
Trapped in The Past: Somewhat obvious trope.
Vanity Publishing: The first few books were published by Baen Books, but the later novels are entirely self published.