As I said upthread, I'm not any sort of expert. And I've never seen a druid in play. So I'm proceeding from the rulebook plus intuition.
Here's the text on p 107 (including the sidebar):
When you call upon the spirits to change your shape, roll+Wis. *On a 10+ hold 3. *On a 7–9 hold 2. *On a miss hold 1 in addition to whatever the GM says.
You may take on the physical form of any species whose essence you have studied or who lives in your land: you and your possessions meld into a perfect copy of the species’ form. You have any innate abilities and weaknesses of the form: claws, wings, gills, breathing water instead of air. You still use your normal stats but some moves may be harder to trigger - a housecat will find it hard to do battle with an ogre. The GM will also tell you one or more moves associated with your new form. Spend 1 hold to make that move. Once you’re out of hold, you return to your natural form. At any time, you may spend all your hold and revert to your natural form.
Animal moves just say what the animal naturally does, like "call the pack," "trample them," or "escape to the air." When you spend your hold your natural instinct kicks in and that move happens. If you spend hold to escape to the air, that’s it - you’re away and on the wing.
(The bolding is not in my copy of the rules. I assume that its absence is a layout error.)
There are references in this to other druid abilities and moves. From p 106:
You learned your magic in a place whose spirits are strong and ancient and they’ve marked you as one of their own. No matter where you go, they live within you and allow you to take their shape. Choose one of the following. It is the land to which you are attuned—when shapeshifting you may take the shape of any animal who might live in your Land.
And on p 107:
When you spend time in contemplation of an animal spirit, you may add its species to those you can assume using shapeshifting.
So it is pretty clear that shapeshifting is into animal forms. Whether dragons count as animals seems like a group-specific, campaign-specific thing. If you're worried about it, because you're not sure how to handle it as a GM, then my default would be to say that it's not, unless it is really central to the player's conception of what his/her druid can do. (And I'd be upfront about this - "I'm not sure how to handle a dragon properly, so can we just agree it's not an animal whose spirit you are attuned to?")