Random anecdote time.
At a used bookstore recently (in Hobart Book Village), my too-young-to-read daughter took a fancy to _The Life of a Bear_, probably because of its 24 illustrated prints involving bears. After realizing it was a ~120 year old book and gently extracting it before she removed one of the pictures, I became slightly fascinated with it.
It appears to be a perfectly cromulent children's chapter book, of similar vintage and theme to _Black Beauty_. Its author was anonymous; in the edition I have, he is simply referred to as "by the author of 'The Life of an Elephant'", the author of which was referred to as "by the author of 'The Life of a Bear'." You can find reprints on Amazon by one of the "reprint any out of copyright books" companies, and a half dozen old editions on ABE.
After much detective work and the WorldCat database, and the assistance of a professional librarian, I did eventually discover the author to be apparently one Sir S Eardley-Wilmot, who was a naval officer at the time (hence the anonymous publishing; it probably didn't pay to be "the naval officer who writes children's books"); he did apparently republish the books under his own name, after he retired, and a third book "The Life of a Tiger" was eventually published, maybe posthumously. He even gets his own Wikipedia article, which mentions the books he wrote on military history, but not his children's offerings.
So anyway, the point of the anecdote.
One point I could make is on the ineffable mystery of why some books are cultural classics and others aren't; why is _Black Beauty_ still beloved by millions, and _The Life of a Bear_ forgotten? Is it purely a matter of artistic merit, do people just like horses more than bears, is there an underlying message to Black Beauty I don't remember since reading it 30 years ago that resonates?
Another point I could make is on the surprising robustness of this print volume, as you mentioned. It has probably been forgotten for a hundred years, not particularly well cared for, just thrown on a shelf somewhere and still in good enough shape.
Or it could be on the inanity of most of the books we are slowly forgetting. Does it really matter if this children's story about a bear vanishes from the Earth? Has our culture been lessened, did we forget something important we once knew, are people less happy for not having read this book?
But what really sticks with me is something completely different: this book is not in the long tail of books. Maybe not the fat head, but solidly in the chubby middle. It received many print editions from multiple publishers on different continents over the course of 30, 40 years. It's in multiple libraries; it's been digitized and is available for reprint at the push of a button. It's probably in the top 10 million of the ~130 million books ever published.
Can you imagine what is in the bottom ~120 million books?