Books on Amazon:
Twin Earth/Chalmers: this is about the distinction between primary and secondary intensions. One could have suspected a priori that water is XYZ instead of H2O. But Kripke shows us that after the actual world has emerged as as it is, we would describe it wrongly as one in which XYZ is water.
N.B.: we would falsely describe them with the primary intension (relative to the actual world) rather than with the secondary appropriate intension.
The Conscious Mind Oxford New York 1996
Constructing the World Oxford 2014