I hugely enjoy David Hurwitz' cd reviews on Classics Today, which are witty, incisive and knowledgeable, with a refreshing absence of undue reverence for the big names in the music industry (record labels and conductors alike) and a total lack of the conspicuous chauvinism that German reviewers display. On the strength of those reviews I have bought many a cd and the choices I made that way have seldom been disappointing.
But on this book: I partly agree with reviewer A. Johnston when he remarks that "Mahler's expressive and easthetic motivations" are not dealt with. What would also have been helpful are extracts from the symphonies' scores to illustrate certain passages (as in Truscott's book on Franz Schmidt's symphonies), so that you can not only hear (on the accompanying cd) but also see the complexity of Mahler's music.
Since in Mahler's music so many things go on, the book is cetainly valuable to the listener since it helps him / her(?) to understand what is happening and how this connects to what went on before and what to expect next. I still find myslef consulting the book every time I listen to a Mahler symphony and I find that it adds to my listening pleasure, so much of what Hurwitz set out to do is achieved.