I think this is extremely common. My DS6 is one of the many who've been reading since they were 2, and he flies through books like food. I think it's perfectly normal and healthy, esp. when you consider how many children in this country spend equal amounts of time bouncing between their tv and electronics.
I wouldn't be at all concerned. 10-15 isn't that much when they're reading a lot, and mine also reads at least 3 hours a day (including about 2 hours before he falls asleep every night). Once they have strong reading comprehension, they can fly through the children's shelves. I wondered about the speed when mine read the first Harry Potter in 3 1/2 hours, but he aced a reading comp. portals quiz afterwards, so I know he's actually absorbing it. In an average summer day, he can polish off 3 regular chapter books in an afternoon.
Likewise, my toddler is a bookaholic and insists on reading about 20 books a day. I'm not sure what we're doing right, but it's great when they love to read!!
