The yellowing is due to acid leaching in the wood pulp paper; it can be reversed or halted for a price, but it's accelerated in conditions of higher humidity.

The yellowing is due to acid leaching in the wood pulp paper; it can be reversed or halted for a price, but it's accelerated in conditions of higher humidity. US hardcovers are supposed to be saddle-stitched and bound, and printed on acid-free paper. UK hardcovers use cheaper paper and perfect binding, as do paperbacks; "perfect" binding is basically cut sheets held together using a thermosetting glue. A lot of late 60s/early 70s UK paperbacks had such terrible glue that they were cracking and the pages falling out within a couple of years.