Here are some links I`d like to share to libraries or similar institutions that provide information about surviving medieval books / bookbinding on the internet:
Geschichtsfenster - Handschriften
Wonderful collection of digitised illustrated medieval manuscripts (mostly from Germany).
Geschichtsfenster - Inkunabeln
Wonderful collection of digitised incunabula/early printed books (mostly from Germany).
Einbanddatenbank (Database of bookbindings)
A searchable database of blindtooled or gilded imprints on 15th / 16th century bookbindings from at least six german libraries (in german)
Library of the University of Salzburg, Austria
Digital collection of bookbindings, Universitäts- und Stadtbibliothek of Cologne, Germany
Highlights from the digital collection, Württembergische Landesbibiothek Stuttgart, Germany
to be continued...