Nobody else said it, so I will- OP should make sure this isn't a 'wet
wall', and he isn't drilling into iron waste stacks or something. Is this
a house or a condo? If the latter, I suspect he is hitting a fire break
wall between his unit and the next one. (Concrete block wall, with furring
and drywall over.) If a house, is this backing up to a chimney stack or
something? Or is this an older house of actual brick construction, and he
is mounting on old-style plaster over masonary work?

This is a multi level condo built in 1954. Brick exterior, I
dont know much more about it. About the fire break wall, got me thinking.
This wall , while not an exterior wall, but does wind up at an exterior
wall.
Plus the other side of this wall , would be a stairwell. I
guess that wall would be a good place to have a firewall. How do I know for
sure, and what do I do if this is the case.
Much thanks to all that have responded, and any further info
much appreciated.
Its just the weight of the bookcase has me concerned, and with grandkids
running around sometimes, I want to make sure this bookcase stays put.
Much regards