by Mark Gavin
The single best book written on learning Postscript is “Learning Postscript: A Visual Approach” written by Ross Smith.
This book was published in March of 1990 and has been out-of-print for a long time. But through the magic of the internet this masterpiece is easily available.
Postscript programming was fairly easy for me to learn; since, I already knew how to program using the Forth programming language. Since Postscript is derived from the Forth programming language; books on Forth are also very useful when learning the programming fundamentals of Postscript.