Well, historically Vertigo has published traditional trades as Volumes, recollected trades or softcover versions of deluxe editions as Books. Not an official way to tell if a collection is a normal sized trade or a fat chunky one, but a useful distinction. Volumes = traditional trades and Books = fat thick trades.
I bought the Kid Vertigo Book One out of curiosity and out of support for the new fat trades. But alas, despite being called Book One, this is very much a traditional trade paperback.
Behold, the treachery!