As far as I know, 'Cossie' is essential reading in any case for any microlight pilot - I've yet to start my training, so can't compare to experience yet.
However, it covers the syllabus, usually with a chapter for each exam, in good detail - it uses good simple analogies to explain concepts, and you have to remind yourself that almost every word that you read forms part of the syllabus and needs to be understood and committed to memory to pass the exams. The pink-shaded areas (and there are only a handful of these) go into more detail than required in the syllabus, for those interested in learning more.
What it doesn't do, is give you a clear checklist/memory map of what you need to know, so you may need other study guides to cement your knowledge. Also, there are no exercises (I lie.. there's one!), so there's a long way to go between reading this book, and being able to practice what it contains.
So what this is, is a detailed introduction. If you're considering an NPPL, and want to see in detail what is involved, or if you're preparing for an NPPL and need an easy-to-read book to introduce concepts and theories to you, then this is it. But you'll need more to be able to use all this in practice - I assume either the school helps you with this, or you'll need some more books.