"Ah, it appears I have some future competition!" Merlin came back to present the book to Luna for her edification and admiration. For a moment, he was on the cusp of just giving her the book, telling her, if you like it that much, it's yours. But... he really liked having his books. And if he kept them, that was incentive for Luna to keep coming back. So he didn't do that, just instead asked if her hands were clean before he let her touch it.
He tilted his head to look at her legs as she did, and shook his head with a sigh. "Those legs, Luna... I've missed seeing them."