It is about the mathematical structure, and how it differs from the classical one. In respect to this there are really some very original and authentic ideas in it, that you will not find in any other book, e.g. the kochen-specker theorem is put in a very clear perspective.

Personally I found it a pity that he waisted half of his book on the easy mathematics of vector spaces, but if you are new to the subject, it seems well explained. Also he avoided touching the harder mathematics. Gleason's theorem (the most important theorem for the qm structure) is only touched. I also skipped the philosopical pages when reading.

But anybody who is interested in the math. struct. of physics SHOULD buy this book because of the rich ideas you will find in it.