Definition: a base or pedestal
This is a big, sculpted stone, onto which you would put a smaller sculpture or bust.
It’s often used in the context of ancient architecture, but you can buy plinths at many home improvement stores.
Why I Looked It Up
I didn’t record the book, but I captured this quote from something:
At the center of a small stage, illuminated in a shaft of light angling through the clouds, a book rests atop a plinth.