"Rare" may be a low print run of some local's memoirs decades ago, or a book on the local railway, a society of firemen, people sent to a world war etc. Not something that excites the world, but still valuable to historians or people interested in local history. Even the guide to a local cave from 1960s might be printed on bad paper (so many copies perished) and contain some technically obsolete details on the then-state of nature, tourism etc.