If you are going to be working with PostScript you will need this book (widely refered to as the "Red Book.")
It is indeed THE reference for PostScript, but the organizational aspects of the book itself are a bit confusing until you understand some PostScript fundementals and have a grasp for some oddball "Adobeisms."
In addition to this book I've always made a habit of leveraging the "suppliments" released for each version of an interpreter.
For example, PostScript level 2 actually consists of several versions of level 2 interpreter (2011, 2015, etc) all of which have associated supplements describing specific features supported (or not.) The supplements are available for download from Adobe's developer side of their web site (they are often far smaller than the Red Book, and absolutely indespensible.)
Additionally, each manufacturer who OEMs an Adobe interpreter sometimes release documentation pertaining to which specific features within an interpreter version are supported, and how.
For example, companies often develop specialized applications built upon frameworks provided by Adobe in an interpreter version. Understanding the customized PostScript commands for these applications enables you to take full advantage of them.
Either way, buy the book. Don't waste your time downloading it. Sure, that's "tree friendly" but we all know it will end up getting printed anyway.
