If you’re a shooter of guns, chances are you’ve tried putting up stuff to check penetration. Books are the target of choice.
And if you’ve done this, you’ve discovered that a book will stop a .22 LR with about 150 pages. As soon as you get into centerfire rounds, they pass through almost any book.
This is such a regrettable, avoidable incident.
If the pages are light enough, you could have a lot of them, and bullet would not get thru them. F'rinstance, using Hard Choices, and you could shoot a BB gun thru a million pages.