The book The Art of "How to Train Your Dragon" contains over 350 art pieces of dragons, the Vikings, and scenery/props. It is available in hardback format only, totaling 160 pages.
The book is an in-depth look at the art that went into the movie, from pre-production sketches to post-production computer visualizations. Many elements that never made it into the final film are present in this, including Hiccup's mother and early designs for Toothless. However, the book isn't cheap; you may get lucky and find it online for $30 or so, but the suggested retail price is upwards of $40. But the money is worth it; you get to see a ton of art for one of the greatest CGI-animated movies of all time!
There are several online retailers that sell the book:
To view current listings on eBay, click this link.
To view the book on Amazon.com, click this link. (several listings available)
To buy the book from DeepDiscount.com (the cheapest one commercially available), click this link.