Here is the three original Postscript Books. They are the classic way to learn PS.

The Blue Book (Tutorial and Cookbook) https://archive.org/details/postscriptlangua0000unse

The Red Book (Language Reference) https://archive.org/details/postscriptlangua0000unse_l1h3

The Green Book (Language Design) https://archive.org/details/PSGreenBook