I remember being surprised when I read an interview with Robert Sedgewick where he listed the PostScript Language Reference Manual on his list of top ten programming books. I've never read it though.
They are also available free online -- "PostScript Language Tutorial & Cookbook", also known as the PostScript Blue Book, is at https://www-cdf.fnal.gov/offline/PostScript/BLUEBOOK.PDF. There is also the Red Book (useful to fully understand the language, to the point of writing a PostScript interpreter), and Green Book (more of a reference book than a cookbook).