I found myself having to write a PostScript template from scratch this week that could accept variables and images and it was mad tough. Here are some the resources that got me through it - thank you random blog writing people.
There's also a sneaky way to get hold of the Big Red Book, give me a tweet for details.
Drawing and stroking a path
Fundamentals: syntax, math, operators
Cambridge University, Engineering Dept
Starlink - Cookbook
Sample Postscript file
Example PostScript files
PostScript command summary
Ghostscript and PostScript language guide