This is a useful, short dictionary, probably worth its modest price.
It is by no means a complete reference work on music notation. Instead, it covers most of the basics, and also a more-or-less random, small selection of less basic topics. This is not a guide for reading music; it is a handbook for people preparing notated scores and parts, whether by hand or by computer. It is unlikely to answer all the questions that would come up in notating any reasonably complicated piece of music. It will suffice, however, to notate a simple piece professionally, in either classical or pop/jazz style.
All of the entries accurately describe standard notational practice. Precise guidance is often given as to exact positioning and size, e.g. whether a given articulation is centered on a note stem or a note head, what angle of beam to use for different intervals, and where to place dots. The sections on topics like meter, rests, slurs, and ties are lucid and thorough.
On the other hand, precise guidance is often absent. For example, correct notation of the treble and bass clefs requires knowing that they are also, respectively, a "G" and an "F" clef, and should be positioned in a specific way to mark those pitches. The entry on "glissando" fails to explain how to notate the precise rhythm of a glissando (in the sense of its duration, and at what point in the measure it ends). The positioning of accidentals in chords of more than 3 notes is explained ambiguously, but not illustrated. No guidance at all is given on how to notate an interval of an altered unison (e.g., a B-flat/B-natural pairing.)
Little guidance at all is given on notating wind or percussion parts.
By contrast, several pages are given over to repetitive explanations of the various permutations of "D.C./D.S./coda/fine" combinations. Complete charts of key signatures in treble, bass, alto and tenor clefs use up more pages (even though the pattern is the same in every clef.) Oddities like flutter-pedaling on the piano damper and heel-toe technique on the organ pedals are illustrated. Overall, the choices of what to cover outside of the basics have a random feel.
Large print is used, which means less material is covered, but subtle details of notation are easily seen.