For now, I have pillaged some useful links to conceal the fact that I haven't written much yet.
This page is moving to a new home (Quite Software have a larger site), though as yet there is nothing much different there. If you are bookmarking it please use the new address http://www.quite.com/ps/.
The word PostScript is a registered trademark of Adobe Systems and their web page has information on that and their many other excellent products. Or, go straight to their PostScript page.
A useful work in progress than may also save me a good deal of trouble is A First Guide To PostScript.
The PostScript FAQ (Frequently Asked Questions) is full of answers, anda very comprehensive book list. At the end of this link is an FTP directory with the FAQ in a variety of formats and arrangements.
Luminous Corporation are a spin off from Adobe. They have lots of useful PostScript information, including detailed descriptions of all PostScript operators (for programmers).
If you don't want to spend good money on PSAlter without seeing the opposition, why not try GhostScript?