Here are a few books I've found to be stimulating:
- The castellated and domestic architecture of Scotland / David MacGibbon, Thomas Ross
- The ecclesiastical architecture of Scotland from the earliest Christian times to the 17th century / David MacGibbon, Thomas Ross
- The castles of England: their story and structure / Sir James Dixon Mackenzie
- The castles and abbeys of England / William Beattie
- Some Account of Domestic Architecture in England: From the conquest to the end of the 13th century / Thomas Hudson Turner, John Henry Parker
*An interesting example of this is the Stokesay Castle, where the only image of the castle with a wet moat is from an 18th century engraving. Compare the pic below with modern photos on the CastleWales.com site:
Another great place to look for public domain works on Castles, etc. is http://www.archive.org.