Excellent, up-to-date coverage of the plants it includes, big gaping holes where common plants are missing (eg. americana, tequilana, sisalana).
Lots of useful information about size, habit, culture, and habitat throughout. Very nice, often glowing pictures. The section on resolving look-alikes is particularly helpful, as is the section listing agaves by size.
If you're looking for your first book on agaves, start with the volume by Irish, which is much more complete. Gentry's book is older and denser and lacks color illustrations but remains the definitive tome on the genus.
I think it's an excellent book, and that the author focused on plants which with he is personally familiar. Other than the popular cultivars, Mr. Starr seems to have encountered all those he writes of ("eighty species, cultivars and hybrids...")in situ, with a travel log aspect that directs interested parties toward their locale. Lots of good sized pics as well.
I agree with Geoff, a comprehensive text would be welcome, but I doubt one is coming in the foreseeable future.
For reasons I still cannot understand, there are very few books on Agaves, despite their obvious popularity. The most surprising thing is the lack of an 'Agave encyclopedia' or some lexicon on the subject. And that is still the case. The information in this book is very well organized and nicely illustrated, with sections on many agave species as well as a few popular cultivars. But it leaves out so many species I am still unclear as to the point of the book. Perhaps it is a books about the author's favorite species. If one has this book along with Irish book on Agaves, one does get a pretty good covering of most of the common species of agave in cultivation, but it is too bad the author did not try harder to create a more complete volume on these fascinating plants.
The book includes a very good section on growing agaves and treating their diseases and problems. It also has a great section on comparing similar looking plants, and how to tell these apart (a section I find particularly useful).
Overall I give the book a positive review with the only fault the glaring absence of so many important agave species. Still no one has written anything comparable (yet).