The Adobe PDF Reference is similar to the Adobe Postscript Language Reference; in that they can both be compared to a dictionary. A dictionary is a document which contains all of the words that can be used in a language; but, it doesn't teach you how to combine those words into a good, well structured book.
PDF is based on Postscript. The documentation for Postscript was released as a set of three volumes.
Postscript Language Reference - Red Book
PostScript Language Program Design - Green Book
PostScript Language Tutorial and Cookbook - Blue Book
These documents are typically referred to by the color name; because, the covers of the books are actually Red, Green and Blue.
The PDF Reference is similar to the Postscript Red Book. Unlike Postscript; Adobe has not released a PDF Blue Book or a PDF Green Book to help teach developers how to construct well formed PDF files.
So why is Postscript important to PDF? Much of PDF is built on top of Postscript; and, since the Blue Book and the Green Book do not exist for PDF; reading and understanding the Postscript documentation is the next best information source available to PDF developers.
The above links can be found on a web site for the "Collider Detector at Fermilab". The lab has an excellent page on Postscript.