Good books on Castles/Forts/Walls?

Mar 2017
West Midlands
I've recently started volunteering in at the Coventry Priory and one of the things that they do is a tour of the remains of the Coventry city wall and the towers. I'd just like to know if anyone here can recommend me a good book on defensive structures in general as I think it would be useful.
Jul 2017
I've been searching as I don't recall ever reading (or seeing) a book on this topic. Walls and Towers: Systems of Defense in Ancient China by Yun Qiao might be worth a read, though. It's obviously going to differ slightly from walls in Coventry, but the premise is the same.

Also, good luck with the volunteering! I'm originally from Rugby, so Coventry is my neck of the woods!