I met today with Ute Neubucher, specialist Art Librarian and Susanne Mihm-Lutz, a lecturer in the Buch und Papier / Artist Book and Paper department at the Kunsthochschulbibliothek of the University of Kassel.
We discussed their library, and they generously took me through some of their Artists' Books collection.
While they have a great assembly of the (now no more) small-run art publisher Rainer Verlag (catalogued here, but reviewed and photographed nicely by Specific Object here), the bulk and most interesting part of their collection is in their collection of unique Artists' Books, stored in these plan/map drawers in a small locked room upstairs.
Acquisitions began seriously in 2003, and the collection now stands at about 120 titles, beautifully catalogued here.
To my delight they have pulled some aside for a closer look.