Well of course he can't read himself, books don't have eyes. Strange, unfortunately, is still stuck in a 'literal book' mindset, failing to realize that Childermass is talking about a person. As such, he still finds the situation quite funny and that smile still plays on his face.

"I assume you've already got ideas in mind of where to find a key?"

Because of course he would. This is Childermass. The man never did anything rash of halfway thought out.