I collect old books, and frequently the paperback binding glue turns to powder, and the book falls apart in ones hands.
However, with care, the pages can be reassembled in the right order, nice and square, clamped between two small sheets of plywood, and the old glue cleaned off the exposed spine with a rasp.
Then a couple of coats of Copydex along the refreshed spine rebind the book. When properly dry the book has now been re-blocked, and can be gently removed from the clamp.
Finally a generous last coat of wet Copydex on the spine is used to carefully float the cover back onto the re-bound block.
I've repaired a couple of dozen paperbacks over several decades, and all the books I have re-bound with Copydex are still flexible and intact. It does not change colour with age, and does not go hard or brittle, but stays rubbery. It is wonderful.