[He sighs, blushing a little more purple, not quite meeting her gaze.]

I wasn't intending to make a habit of staying up all night reading, I assure you. [Oops? ...Maybe that's all it is, him getting sucked into reading a good book. She's surely spotted him reading stuff from the library before anyway; it's entirely plausible!

More quietly,] I'm nearly done with the book I've got right now.