"History of Castles: Fortifications Around the World" is absolutely astounding. I found this oversize 2001 book while browsing randomly through the 710s and 720s (Dewey Decimal) library aisles.
The text is extremely well-written, and is never boring or condescending, even though Gravett goes very in-depth on his subject matter.
The book is filled with hundreds of amazing sketches, photographs, and color plates of castles and fortifications from around the world, many of which I had never seen before. It ends with a fascinating one-page glossary of castle-building terminology.
I am busy right now, selecting a few of the book's photographs which I will attempt to sketch for my own enjoyment. It is painful to know that I must return this beautiful volume to the library in just a couple of weeks.
I could continuously read this book over and over again for many years, and never be bored.