The extensive collections of letters in the Royal Library are primarily located in the Manuscript Collection : letters from the 1600s up to today, and from both prominent and lesser known Danes.  Most of the letters are from well-known persons from Denmark or abroad and by far the greater number are from the 19th and 20th centuries.

Search for letters in the Letter Database and in REX, or ask the Department

Though a number of important correspondances have been published, in particular by the Danish Society for Language and Literature, e.g. Adam Oehlenschlæger, Hans Christian Andersen, Georg Brandes and Martin Andersen Nexø, by far most of these close to two million letters can only be accessed in the Royal Library's Research Reading Room. 

Other departments at the Royal Library also have isolated collections of letters. The archive of Chief Rabbi David Simonsen in the Judaica Collection contains a considerable number of letters, as does the Holger Laage-Petersen Hans Christian Andersen Collection in the Legal Deposit Collection contain a number of letters to and from Hans Christian Andersen.