A book that sums up Gettysburg (among other modern) battles that were decisive and could be won by the historically losing side. While it is not a big book it does provide the basics from a new perspective.
http://books.google.se/books?id=I7NlpwAACAAJ&dq=Sun+Tzu+Gettysburg&hl=en&sa=X&ei=O83TUZyeLMbNswa7iICoDw&redir_esc=y