I was amazed when reading this book how many easy-looking victories white could fight out with this opening, mostly following the same pattern (Qe1-h4, f5, Bh6, Ng5). You can hardly evite the conclusion that the gpa is an interesting, and by far not drawish alternative to 2.Nf3. As for the black side, the more useful it should be to dedicate some time to this opening.
As for the book itself, i can only tell positive things about it. It is slightly in favour of white, of cause, as the auther uses it regulary in his practice (wich in my view is a plus for the book), but he is far from being impartial or ardent to underestimate blacks possibilities to equalize.
The author has been very diligent in his work, used e.g. mega-database 2007 and a lot of other opening books. He collaborated namely with N. Palliser, who had published a big chapter about the gpa in his recent book, and it seems that jones found some improvements.
Very positive in my view is the fact that, not like many other, mainly british opening books (i am german, elo 2000) it does not present you just a collection of annotated games, but really, say, the full theory; of cause, plenty of games are included.
The layout is fine. A good idea is also to put two diagrams together in a row, this saves a lot of space. As you have a double-diagram at nearly every page, having a 1-column-text as here, you could suggest the book to have been about 30 pages thicker (and consequentally more expensive) with the traditional layout.
To sum it up this book is a good deal and worth its money, and of great interest for the attacker as well as for the defender. For the latter a little tip: think about 2.-a6 to confront 2.Nc3! - There is one chapter about it in the book (pointing out that only f4/g3 could give white an edge), plus 2.-d5 3.ed Nf6!? to confront 2.f4, then you will hopefully not have to worry about the gpa any more! :)