> But for what? Most heirloom hardbacks are better-than-average novels for well-heeled fans and collectors, not a complete guide to rebuilding civilisation.

They're not a guide for rebuilding civilization. They're something that can last through whatever dark time you're considering in some semi-forgotten corner, so the next civilization can learn about and study what came before. And honestly, they don't need to last the whole time, but rather only until thins recover enough that you start to have copyists, etc.