What’s the Oldest Book on Your Keeper Shelf?

I got into an online chat with a writer friend the other day about keeper shelves, and an interesting question arose: what was the oldest book we had on our keeper shelves? Which old favorites did we keep coming back to, over and over again?

And why?

After some discussion, we determined the word “old” in context with keeper shelves had several definitions. For the remainder of our chat, we focused on two: our oldest book by first copyright date, and our oldest book by physical age.

My oldest...