Every worksheet comes back with a dragon on it, and your child has firm opinions about which ones are strongest. Any one of these books feeds the obsession, so pick by your child's age and by how much dragon they can currently take.
A dragon obsession is never just one thing, which is why this is a shelf. One child wants dragons to be funny. Another wants them small enough to hold. Another wants a book that agrees dragons are real and tells them exactly how a wing is built. And another wants five dragonets in a war. Look at the age on each one, then look at how much your child actually wants to be scared, and hand over the one that matches. If it lands, come back and take another off the shelf.