How tall could a bookshelf be? Maximum I would think five feet.
Lupaglupa has put their finger squarely on the variable that can be calculated accurately. It is, literally, a high school physics class exercise to calculate the velocity & kenetic energy of a cannon ball of X weight dropped from 6' / 2 meters on down to 3' / 1 meter. What we cannot know is the nature of the flooring impacted in the example.
For what it is worth I dropped an 32 ounce framing hammer onto a 3/4" red cedar floor from about 12 feet / 4 meters.
The sound porch decking was put down in 1883. Much to my chagrin, the hammer hit a knot. The butt lapped 6" wide board broke along typical red cedar grain lines. While the hammer did not go through, the 18" long splinter did. Retrieving it was… well… a process.
I was removing a walk board scaffold, the last task from putting up a 50' X 18' tongue & groove ceiling.
Of course, the repair is right next to the front steps & annoys me every time I pass by it.
By contrast, at (+/-) 800 yards I have seen a 12 pound cannon ball grazing shot bounce off a 3/4" sheet of plywood decking used as a target. Rifle bolts smacked right through it. Variables, variables… variables.
My experience has been that if you hit a board exactly right on a knot, it will break along grain lines. Sound modern plywood flooring requires something more violent than a 12 pound weight dropped from a shelf to be penetrated.
On the other hand, and as you point out, if it were, say, a 12 lb ball for a M1857 Napoleon, I would have moved out well before that happened. 
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