White Paper: Scripting: Higher Level Programming for the 21st Century, John K. Ousterhout, 1997. This document is available at http://www.sunlabs.com/~ouster/scripting.html.
Blue Book: PostScript Language, Tutorial and Cookbook, Adobe Systems Incorporated, Addison-Wesley, 1985.
Red Book: PostScript Language, Reference Manual, Adobe Systems Incorporated, Addison-Wesley, 1985.
There is a good chance your system administrator has a couple of these books; in the good ol' days you got the pair when you bought a PostScript printer.
DSC: PostScript Language, Document Structuring Conventions Specification, Adobe Developer Support, Adobe Systems Incorporated, 1992. This document can be found at the Adobe web site http://www.adobe.com/.
PostScript by Example, Henry McGilton and Mary Campione, Addison-Wesley, 1992.