Oxford Essential Dictionary of the U.S. Military

This is much more broad than deep. Extensive amount of entries include information on history and mundane basics such as ranks and types of units.

Dictionary of Modern War

By Edward Luttwalk and Stuart L. Koehl

This volume is more like an encyclopedia than a dictionary. It gives good coverage of equipment and many military concepts. It has both U.S. and foreign entries. One drawback is that was published in 1998.

Pen & Sword

By Ed Offley

A good introduction to military beat reporting written by a Navy veteran of Vietnam who has covered defense and military issues for more than two decades.

Stolen Valor

By B.G. ‘Jug’ Burkett and Glenna Whitley

An interesting book detailing the phenomenon of people claiming military or combat service or extraordinary heroism and medals when, in fact, they did not perform such actions. A must read for any military reporter.

Also deals with how to expose such frauds. Most of these people made claims about Vietnam; the phenomenon will repeat itself with the wars in Iraq and Afghanistan.