The book on the right had no back at all. Probably it lost it's back in the years of time and then a rebacking was done (probably in the 19th century, proving the remains of leather and glue on the original cover-leather). This rebacking did not withstand the ages and had disappeared too.
The bindings (alum-tawed leather) were broken on several places and had to be renewed. For this I used the same material as the original and the bindings were laced into the boards and pegged with wooden pins. At the head and tail of the backbone of the book special headbands were reconstructed. They consist of a leather core which were wrapped up with cord (linen thread). Later on the leather that will form the new back is folded around these wrapped-up cores and then stitched through the leather with thicker linen thread.
The new back that was applied was made of vegetably tanned calf leather. In this case it was dyed to match the colour of the remaining leather. The edges of the original covers were pared on the inside to make a regular transition. Then the new back was placed with starch on the backbone and on the wooden boards.
The bosses (cardinal-hat) on the covers were reconstructed from the imprints in the cover-leather (the diameter could easily be deduced) and the shape from similar contemporary bindings.