The Internet Archive features the full text of a very large number of books on military history topics. These works are either out of copyright (in which case downloadable versions in PDF and other formats are often available) or can be borrowed for short period through the Open Library (free account registration required). The site's search functionality is somewhat limited though.
To help address the difficulties of finding resources in the Internet Library, this is a user-generated list of works members of the project have located and found to be useful. By its nature, the list will always be incomplete.