Technically, there are fifteen books in the How to Train Your Dragon Series, since the loose continuity style of the first few books means that Day of the Dreader and How To Train your Viking could feasibly slot somewhere between How to Train your Dragon and How to Break a Dragon’s Heart. And there’s nothing in the main twelve implying the events of The Viking who was seasick didn’t happen when Hiccup was a little kid.
Oh yeah, Cowell wrote three little side-books for World book day. The Day of the Dreader, How to Train your Viking, and an (in)complete guide to Dragons (think the little blurbs about Dragons that are scattered throughout the books but it’s a whole book of that). Whether or not you consider them canon is up to you.
Before she wrote HTTYD, Cowell wrote a book for little kids/toddlers called Hiccup: The viking who was seasick. This one is so dubiously canon that it could feasibly be a prequel to both the Books canon and the Dreamworks canon. This is also the only one with no Dragons in it.
