Leo doesn't have any special attachment to the book Donnie picked, and he has nowhere to be, so he lets Donnie hunt as he pleases, following along and giving an encouraging comment any time Donnie looks back at him.

After a couple minutes he leans back against the shelves to let Donnie search, flipping first through the space book, and then pulling out his Houdini book to start looking at that. Donnie seems pretty happy looking at the plant books so it's probably fine to look away from him, right...?