Erik Kwakkel shows this picture from the Bodleian Library via Twitter. The Bodleian caption describes it thus “Transparent vellum binding by Edwards of Halifax, c.1785. Members of the Edwards family took out a patent in 1785 for the process of rendering vellum transparent by soaking it in pearl ash and subjecting it to high pressure. They also made a speciality of painting landscapes on the fore-edges of books: this volume shows a painting of the falls of Tivoli, but it is only visible when the pages are fanned out.” You can zoom in on the image at the Bodleian’s Luna site, here. More information about the process and its patenting by Edwards of Halifax at CoolConservation. This piece expresses uncertainty as to whether the patent covered both the treatment of the vellum or the method of painting on the back, or only one of the two.
Booktryst has a story about Cedric Chivers of Bath who version of this process was called Vellucent binding. This story has several images showing quite elaborate transparent vellum binding. It looks to me that the processes differ in that the Edwards of Halifax process involved painting or printing the design in reverse on the back of the translucent vellum, lining that with a white sheet, and then binding the pair onto the boards. Mr Chivers’ Vellucent technique had the design painted onto the backing sheet, then covered by the vellum to make one “indisseverable” sheet.
While these binding processes no doubt have their own special character we would recognize them today as lamination applied to a paper over boards, particularly a matte lam. Designers seeking the translucent effect can now use a paper substitute, though as far as I know nobody has been printing on the back of such a paper.