Ms Gould's lengthy reference has rightly become the bible of music notation, the standard that most people in the industry (music engravers, publishers, and composers) regard as most comprehensive and most authentic. Like any style guide, it is not officially sanctioned to the point that it is beyond question, but, as with language style guides, it is taken seriously because the author has both personal experience dealing with the issues (widely regarded for her own work as an engraver) and because recommendations (not rules) are so well thought out and clearly argued with strong reasoning points. Any notator of music who knows better on any given issue is, of course, free to do what he or she pleases.
By way of background: I have been close to classical music my entire life, was at one time a composer of complex music, and I spent twenty years of my working career doing mostly music engraving for a living. (The term "engraving" is a misnomer, as it's been a very long time since anyone has etched metal plates to set music, but the term has stuck in the industry even up to today, where almost all printed music is set electronically.) As a result, I considered myself somewhat of an "expert" on the subject of music notation when I first opened the cover of Ms. Gould's book.
But I'd failed to recall that the problems of music engravers extend way beyond the principles of how to read music that everyone's old lady piano teacher taught him in early grade school years. Engravers must also deal with problems of layout (for page turns and other concerns), part extraction, cueing, coordination, and today an enormous array of special techniques that have arisen in the written music of composers during the last fifty years or so.
Music notation is an art form, very much intrinsic to music composition, but also a discipline worthy of the highest respect on its own. It has been my experience that Behind Bars, even as a fairly new publication, has earned itself a place at the top of publications that are dedicated to explaining the art's current practice. The book is as close to complete as one could imagine and extremely well written and edited (in British English style -- I'll admit that I still have not been able to adjust to the use of stave rather than staff for a single instance of the horizontal lines that music is written on). Of supreme importance, the graphical presentation is superb, with examples immediately following (or sometimes preceding) examples, or in some cases well cross-referenced. There is, of course, an extensive index, a feature that is indispensable in a reference work of this type and magnitude.
It took me a long time to get through this book, reading just a few pages at a time, but I did read every page. I'm sure not everything stuck in my brain, but at least now I know where to look for advice if and when I have questions about problems in music notation.