Astronomical observatory publications include the work of local astronomers from observatories around the world and are traditionally exchanged between observatories through their libraries. However, large collections of these publications appear to be rare and are often incomplete. In order to assess the unique properties of the collections, we compare observatories present in our own collection from the university at Copenhagen, Denmark with two collections from the USA: one at the Woodman Library at Wisconsin-Madison and another at the Dudley Observatory in Loudonville, New York.