Anatomy of the Castle by John Gibson is perhaps the best book describing Castles aimed at the general reader that I have ever read.  The author manages to make the somewhat technical and dry language of describing castles and their construction lovely and entertaining.
It is a coffee table sized book that is jam packed with beautiful color photos of castles from all over Europe and the Middle East.  There are 200 pages with a glossary, index, and bibliography.  It is divided into 6 chronological chapters with a lengthy introduction that describes the development of the art of fortification up to the development of the first castles.  He also includes a chapter describing what living in a castle must have truly been like.  The glossary is short but helpful as it includes all the technical terms that are easily misused.
John Gibson has produced work about castles and their construction that is both informative and entertaining.  He deftly covers the castles invention and development over a period of about 1,000 years and ties the castle into both hat came before and what came after in the art of fortification.  Along the way he dispels some myths about castles, such as that they were dark dank places or the opposite that they were full of light and warmth.  He gives the lie to both notions and establishes that the truth lay somewhere in between.  He also points out that dungeons as described in popular literature did not really exist although there were some places in castles that were used as prisons including entire castle at times.  What was good for keeping people out was also pretty good at keeping them in when used for that purpose.
Aside from the quality of the photos the thing about this book that I enjoyed the most was the quality of the writing.  I never got bored while reading this book and the illustrations are well placed to illuminate the text.  There are several fold-outs of significant castles that illustrate stages in castle development.  This is a highly enjoyable read and I highly recommend it.