There are a lot of papers used specifically in bookbinding. There is repair tissue, a strong pure Gampi paper. A pure Kozo used to laminate thin papers and fabrics to make a suitable covering material, especially silk, as well as strengthen edges and torn pages. Sometimes used to mend inner and outer hinges hinges. It can be dyed to a suitable colour.
There are endpapers from plain colours of different weights to richly patterned Japanese, Himalayan and other handmade patterned papers.
Acid free Bookbinders board is available in 2.4mm