By 'unpublished letters' we assume is meant, not published in Letters. There are still many Tolkien letters which have never appeared in print, in whole or in part – more than we can know, as we've not had access to the preserved Tolkien family correspondence; and then there are many letters which have appeared only in part, e.g. in auction or manuscript dealers' catalogues, which would come under Morgan's heading of 'hard-to-find'. As Garm very kindly says, we included parts of many previously unpublished letters in our Companion and Guide, over a hundred of them, and also quoted from extracts previously published in catalogues or elsewhere. The copyright statement at the back of either volume points to sources, supported by our bibliography of works consulted. Wayne's Descriptive Bibliography, Section Dii, is also a useful list of separately published letters or extracts up to mid-1992.

There are also letters recorded in the addenda and corrigenda to the Companion and Guide, at this site.

Wayne and Christina

Sincere thanks for all the answers!