Just FYI: A summary of a good book:
http://arxiv.org/abs/1101.1063
Progress in revolutionary propulsion physics; Authors: Marc G. Millis; (Submitted on 5 Jan 2011)
.
And another related paper by the same author:
http://arxiv.org/abs/1101.1066
Energy, incessant obsolescence, and the first interstellar missions