Millions by Frank Cottrell Boyce
This is a novel for kids 8 and up, supposedly, though it seems too mature for that young. I really liked it. The main character is obsessed with Catholic Saints and being good, and sometimes he sees and talks to them. There's also a bizarre portrayal of Mormons in their neighborhood (the kid's intrigued, because they're Latter-day Saints, you know), which makes me really wonder if the author has met some LDS missionaries who were kind of odd. Or maybe he just made it up. Or maybe missionaries in England go by their first names (all Biblical) and live in suburban homes in threesomes eschewing material possessions.
There's a very good movie version of this, too, but it also may be too mature for young kids. We watched it with ours, but it won't be one of those that gets oft-repeated viewings.