Read the documentation in this order to easily learn postscript:
Paul Bourke's excellent tutorial: http://paulbourke.net/dataformats/postscript/
Blue Book, first half, the original official tutorial:
Green Book, how to use postscript effectively:
Thinking in Postscript, 'nuff said: http://wwwcdf.pd.infn.it/localdoc/tips.pdf
Mathematical Illustrations. Start small, build big. The math behind Bezier Curves. The Hodgman-Sutherland polygon clipping algorithm. Affine transformations and non-linear transformations of the path. 3D drawing and Gouraud shading. From the preface:
Which [of the many tools to help one produce mathematical graphics] to choose apparently involves a trade-off between simplicity and quality, in which most go for what they perceive to be simplicity. The truth is that the trade-off is unnecessary — once one has made a small initial investment of effort, by far the best thing to do in most situations is to write a program in the graphics programming language PostScript. There is practically no limit to the quality of the output of a PostScript program, and as one acquires experience the difﬁculties of using the language decrease rapidly. The apparent complexity involved in producing simple ﬁgures by programming in PostScript, as I hope this book will demonstrate, is largely an illusion. And the amount of work involved in producing more complicated ﬁgures will usually be neither more nor less than what is necessary.