In 2014, Cressida Cowell released "The Incomplete Book of Dragons" in the UK and The Complete Book of Dragons in the US. Both are identical except for the In-. These books detail many dragon species seen in the How to Train Your Dragon Books, as well as some that aren't seen in the series. Also introduced in these books are a classification system for the Book dragon species. Any classification is not really mentioned in the actual series. It is possible that the author introduced a classification system not only to organize the plethora of species, but also because classification is very popular in the How to Train Your Dragon Franchise.