My wife teaches highschool mythology and she suggests
http://mythman.com/ for some info on mythology. Click on "HOMEWORK HELP CENTER" and there are a lot of things categorized. As far as weapons and such, it would really depend on the era and location you are wanting to look into. I know you said "midevil," but a lot depends on location and even time. The midevil time period was broken into 3 different times i think. But check out
http://www.middle-ages.org.uk/middle-ages-timeline.htm
I know they aren't books, but there a lot of things to read there!
My green thumb grew the tree my Trojan War horse was crafted from. With roses in our pockets we rally round the tombstones. Ashes to ashes, we all fall down.