Godel, Escher, Bach: An Eternal Golden Braid has already been mentioned, but bears repeating. Godel's Proof by Nagel and Newman makes a good companion to it. Finally, How to Solve It by G. Polya will help make up for the deficiencies in modern mathematics textbooks. I know I wish my mathematics instructor in high school had given me Polya.
Castles and fortresses were completely negated by modern artillery and war machines. A decent fortification would be an immense advantage against a mob armed with pistols and rifles. Just sayin'.
Solutions are obvious if one only has the optical power to observe them over the horizon. -- K.A. Arsdall