askchapo

Mainly through the Catholic church, as monks were the main copiers of texts. Monasteries were most likely to have copies of history books but often local churches had their own libraries (this is fairly unique to England among western Europe at the time). As far as I understand if you wanted to learn something you would ask a member of the clergy for help, and they would inquire on your behalf to local churches, monasteries, and aristocracy's libraries. If they found a book on the subject you wanted you would usually travel to where the book was kept and a priest or monk would be in the room with you while you read it (reading silently was a rare practice at the time, so they could monitor how you were reading the text) They would also usually try to get anyone with this level of curiosity to join the church professionally

If you were a landholder your priest might be loaned the book and he would bring it to your house, watch/listen to you read it, then take it back to the church for the night.

This time period is also when Oxford and Cambridge universities become prominent so they might have been a resource available but I'm not sure how accessible they were to normal people