I've just completed the first 30 stages of this outstanding trek.
First - is the walk worthy of your attention? Well, experienced guide-book writer Gillian Price, an Italian resident, calls the GTA "probably Italy's most rewarding long-distance walking route" in her introduction and it lives up to the hype - in terms of landscape, culture and wildlife it was an unforgettable experience. But almost unknown to English speaking walkers - hut guardians told me than they only see a handful on the route each year. This is baffling - access is easy from the UK (you can be on the route within 24 hours of leaving home) and there can be few more exiting mountain experiences in Europe at such an accessible level of technical difficulty. The quality of the route is remarkably sustained and even the lower-key link days are worthwhile. Don't be put off by the underwhelming cover photo - it undersells the experience on offer. So the route comes highly recommended!
But just one word of caution - I would disagree that you don't need "any special mountaineering expertise" as claimed in the Book Description - you should have some all-round hillwalking experience. While there is no technical difficulty, parts of the walk are quite exposed and in poor visibility a few of the stages require navigation skills.
As for the guide Gillian is a convivial companion, giving you just enough information on the routefinding, accommodation, culture and natural history while still leaving the book relatively portable and easy to use in the field. There are times when you'll wish she was a little more precise about the route - particularly when entering and leaving settlements, where the waymarking tends to become more sketchy. But combining the guide with maps and waymarking I was able to follow the path without any significant problems. The information is becoming a bit dated, but there's a blog on the publisher's website where readers have submitted updates, and I found that all the essential information is covered.
As for alternatives, if you speak German you could also access Werner Bätzing's 2-volume guide, which covers the many GTA variations not explored by Gillian Price. This a beautifully produced magnum-opus and users told me it is very well done. The walk is much better known in the German speaking regions, but they go from north to south, while Gillian's guide takes the original Italian direction of south to north. To me, this makes more sense. The sun is at your back rather than in your eyes, and the walk is a crescendo from lower to higher peaks rather than a decrescendo. There is also a new 2 volume guide in Italian by Matteo Graziani which covers the route from south to north, though I can't speak for its quality.
So to summarise, a decent guide to an outstanding experience, and your only English-language option.