For a really good book on Castles check out "Castles, A History of Fortified Structures Ancient, Medieval and Modern." Lots of information and some really good illustrations as well. Castles seem to get passed over frequently in the various games. In watching the movie Elizabeth this past weekend, which takes place largely in a Castle I couldn't help but notice how really cool the setting was. We've done a little exploring with Castle here at TLG, with most recently Robert Doyel's Engineering Castles but its a subject that wants for more.