In 1985, the Darwin Correspondence Project produced its first publication, A Calendar to the Correspondence of Charles Darwin: 1821–1882 (New York: Garland), which contained a detailed summary of every letter Darwin was then known to have sent or received. That edition is no longer in print, but a revised and updated edition was published by Cambridge University Press in 1994 and the entries in that form the basis for most of the letter summaries on this website. The printed Calendar is still a standard reference work for Darwin scholars.
In 1996 a separate calendar was published pulling together the details of Darwin’s correspondence with German naturalists (Charles Darwins Briefwechsel mit Deutschen Naturforschern, Marburg: Basilisken-Presse).
The introductions to the printed versions of the Calendar give a good insight into the process of collecting and organising the wealth of Darwin correspondence.