Something made me reread these decent childrens' fantasy books the other day. I remember loving them as a kid, liking them as a younger adult (younger than I am now)and now I just sort of like them.
The first is "The House With the Clock In Its Walls" and is followed closely by "The Figure in the Shadows" and "The Letter, the Witch and the Ring". There are several more and then there are the Johhny Dixon books and the Anthony Monday ones. I haven't read any of the second two series but I imagine they pretty similar to the Barnavelt books. Somehow, whether by accident or happenstance the hero gets caught up in dangerous supernatural situations. That's really about it.
There's great bits about Lewis and his worries as the geeky fat boy and his relationships with his friends but after the second books things get a little too samey.
But you know what? They're real short, you can read them pretty quickly and they're not that bad. Just don't read them all at once.
I also found this cool site about Bellairs and his books. Very well done with some great pictures. John Bellairs Site