I hate to be a wet blanket on an otherwise positive thread (at least through the first six or seven pages that I read), but I'm a tad underwhelmed. The art is decent enough, although for a full page I would have liked to see a scene - a full page of just the monster might end up feeling a bit sparse in the actual book. The sketches are a nice idea but don't really add anything to the main picture. And, to be honest, I'm simply not that fond of the way they depicted the Tarrasque - too smooth and almost rubbery. I would have liked to see more sharp edges, something more kaiju-esque.
Secondly, without a third page it is a very "statty" and overall empty entry. I'd love to see a third page with "Tarrasquean Lore" - examples of tidbits about the creature, stories, and possible ways to use it in your campaign. Flesh it out a bit! Who knows, maybe that is there, but I'm doubtful.
That said, I like the look of the pages themselves - the large "T," the background wash, and the small "T" in the corner for reference.