You can stack the bookcases. None of mine have anything touching the tops of the lids as if it were a paperweight. However, I do stack cans in the pantry, but again there is nothing pressing down on them that would create that much psi on any lid.I saw a study how stacking them (second jar on lid of bottom jar) can compromise the seals and give "false seals" because it won't pop up if the seal breaks due to the jar acting as a paper weight.
So pretty much there's no winning.
I agree on not letting jars or cans getting too hot or cold! There were some cans accidentally left in a camper over winter. I would not sue them as I would be afraid they were compromised from the freezing and thawing.