Paper quality might help with periodisation. I'm studying to be a librarian, and it shocked me to read that older books are not necessarily more fragile.
Specifically, in the mid to late 1800s most printers and publishers began to use a new type of paper of much lower quality that will become brittle much more quickly than the paper of most books published before the mid 1800s. (the technical term is high acidity). This is still true of almost all newspapers and cheap paperbacks.
There are exceptions, of course, but it's surprising to realize that for this reason a book printed two hundred years ago or more is likely to be in better condition than one printed only one hundred years ago.
An extreme example from the magic world: I recently had the privilege of carefully handling an original 1584 copy of Reginald Scot's _Discovery of Witchcraft_: although the pages hade faded slightly they were very readable and they were not at all brittle. I was amazed, actually.
Although acidic paper began to be used in the mid 1800s, the fact that this was a problem wasn't realized until many years later, and now publishers have begun to use paper that is more durable (better, less acidic, more alkaline)in at least some of their books.
I don't know if it would provide some sort of cut off date for rarity, though it might help.