reference manual: https://www.adobe.com/products/postscript/pdfs/PLRM.pdf

tutorial and cookbook (blue book): https://www-cdf.fnal.gov/offline/PostScript/BLUEBOOK.PDF

language program design (green book): https://www-cdf.fnal.gov/offline/PostScript/GREENBK.PDF

Not the OP, but "Thinking in Postscript" is often recommended and available freely.

http://w3-o.cs.hm.edu/~ruckert/compiler/ThinkingInPostScript...

Depends. How do you feel about math?

http://www.math.ubc.ca/~cass/graphics/manual/