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...