It can be quite significant. Authors' letters to their editors and the like often turn out to be important sources to later literary scholarship. Now, this migh

It can be quite significant. Authors' letters to their editors and the like often turn out to be important sources to later literary scholarship. Now, this might be somewhat less of an issue for the most modern authors, now that so much more stuff is written down, but for a notoriously secretive author like Thomas Pynchon for example any letter is likely to contain invaluable information.