Many people may already know about these two links, but if not, please check them out. One is the U.S. Army Center for Military History. This site has a ton of pdf files, including good pdf files of the Army "Green Books" series on the History of the U.S. Army operations in World War II. There are numerous other monographs and books on subjects ranging from our Revolutionary War in the 1770's to current operations in Iraq and Afghanistan.
The second link is the U.S. Marine Corps History Division Publications page. At the bottom of this webpage is a list of links for time periods that contain tons of USMC reference material. The World War II and the Korean War sections are especially useful. But there are tons of downloadable books and pamphlets in PDF form that will enhance anyone's reference library.