Are these book cases 1) mounted high on the wall or are they 2) on the
floor, such that the top surfaces are used table-like/countertop-like,
hence their being scratched?

I'm thinking, just a sheet of 1/4" or 3/8" luan, finished nicely
(durable finish), would do fine, with simple molding around the
edges. Any future damage can be more easily repaired or replaced.
Ceramic tile, directly applied onto wood, doesn't sound like a secure/
fail-safe surface, to me.

Sonny