Hold up, animal glue isn't some magic fix that stays flexible forever. It can actually get brittle and crumble over time, just like bad PVA might crack. I've repaired plenty of old books where the original animal glue gave out. For stuff you want to pass down, I mix a good quality PVA with a little methyl cellulose. It keeps things from getting too stiff and the steps are still simple. Just don't buy the cheapest PVA you find. That mix has held up really well in my projects for years now.