Whoa. What a difference! Same publisher/printer/year/everything. Same library binding. Same library building, even.

Very, very different results.

The copy on the bottom is very, very brittle and brown. The copy on the top has paper that is still nicely flexible and white. 

It’s possible the copy on the bottom was kept in a big city, or someplace where it was exposed to more air pollution than the copy on the top. This is particularly likely if it was gifted to the library after being owned for several years by its original owner first and not bought BY the library for the collection.

Another explanation is that the copy on the top was library-bound much earlier than the copy on the bottom. If the copy on the bottom had a very acidic original cover, this could have hastened the embrittlement of the paper. However, in this case this is not the cause, as these items were bound at the exact same time, according to the bindery shipment sticker in the back.

It’s also possible that the printer switched to a different kind of paper halfway through their print run, so the bottom copy may be a different type of paper than the top copy.