That's a tough one. I asked the same question a while back about removing ancient, dried-out tape from antique film holders. I ended up using mostly elbow grease and a putty knife. Solvents of any kind did little good. I didn't want to use water-based solvents for fear of warping the thin strips of wood, but that's what I ended up using. The tape seems to have had a protein-based glue on it. What worked best was a water-based label remover I found at Home Depot. Something like a wallpaper stripper that has some enzymatic action might work, but those are water-based too. Once you get a corner of the leather up, you might find it will peel right off if it's not too brittle.
Peter Gomena