Thomas Asbridge has a good one on William Marshal. It's pretty short, it's well written, and has good notes if you find an area you want to learn more about. Marshal is a goods subject b/c he's got a Forest Gumpy quality for that period of England. He was around all the big players, went on crusade, was a popular tourney knight, etc. It's called The Greatest Knight. It's a great place to start for English knights. https://www.goodreads.com/book/show/23456467-the-greatest-knight