Elements of Military Art and Science&#13;Or, Course Of Instruction In Strategy, Fortification, Tactics Of Battles, &amp;C.; Embracing The Duties Of Staff, Infantry, Cavalry, Artillery, And Engineers; Adapted To The Use Of Volunteers And Militia; Third Edition; With Critical Notes On The Mexican And Crimean Wars. by Halleck, H. W. (Henry Wager)
 is a Project Gutenberg book, now on Github.