Book limits already?

I know both Dave and I loved to read when we were little and I remember more than a few times my parents had to come down and tell me to turn off my light, stop reading and go to sleep! However, I didn't expect to have to do that with my own kids until they could actually read. Sam adores flipping through his books at bedtime and we've had to start to limit how many he's allowed to go through or he'll stay up for an hour or more. He points at all the pictures, maybe counts various things, talks to himself or tells himself what he remembers of the story. It's very cute, but amazing to me that books have such a draw for him before he can even read the words!