I never bought it actually. The trouble with subjects like music is that each revision guide will say different things. For example, the CGP guide last year said something like "Electric Counterpoint is in D Minor", yet other revision guides would say it was in C Minor, or E Minor. The same thing happened with Grace. My teachers ran an intensive revision session one Saturday morning, it was very helpful and they gave us a 14-page booklet with each set work condensed to the main points under subtitles of 'Harmony' 'Melody' 'Rhythm' etc. Understanding pieces of music under those headings has actually proved helpful for my Music Technology A Level which I have an exam for in a few weeks.
There's a lot of conflicting reports, so it's important to get a guide that agrees with at least 2 other sources, and there's no harm in speaking to teachers for help! Think about it yourself though, does it sound Major? Does it sound Minor? Also, it's helpful to learn the chord sequences for songs like All Blues and Moby's one. They like to ask questions on that.
Hope this helps!