Carroll heads up the Legacy Project, which collects and preserves wartime letters. While there is no complete online digitization project for the archive, there are a number of web sites which exhibit letters from the project:
In partnership with the Gilder Lehrman Insitute of American History, the Legacy Project is exhibiting letters and audio recordings of the correspondences .
PBS produced a critically-acclaimed documentary titled "War Letters," based on Andrew Carroll's national bestseller of the same name, and letters featured in that program can be seen by clicking the link that says "Featured Letters." PBS also provides a brief bibliography related to wartime letters.
History Channel also produced a documentary, "Dear Home," based on World War II letters in the Legacy Project's collection, and has made some of these letters available.
Carroll has edited several previous collections of letters in book format, among them are:
War Letters: Extraordinary Correspondence from American Wars (New York: Scribner, repr. 2002)
Behind the Lines: Powerful and Revealing American and Foreign War Letters -- and One Man's Search to Find Them (New York: Scribner, 2005)