One, my 8 year old, is an uber-reader. Sometimes he goes in fits and starts, but most of the time it's one of his favorite activities. Here is a list of what has kept him busy in the past few months:
The Hardy Boys - he's on #5 and loves them - he's constantly trying to explain the plots to me. Now I know how my mother felt when I read Nancy Drew as a child.
The Spiderwick Chronicles. I can't say much about these, as One reads them at school, where his classroom has an entire set. He loves them, and loves discussing them with his friends.
Bunnicula - ditto on this one - it's a classroom favorite.
The Roman Mysteries series by Caroline Lawrence. One is a history fanatic so these are great for him.
Detectives in Togas and Mystery of the Roman Ransom by Henry Winterfield. So sad there are only 2!
How to Train your Dragon by Cressida Cowell. This is also great for read-aloud for younger boys. We read it on vacation 2 summers ago and almost fell off the bed laughing each night.
Exiled: Memories of a Camel, by Kathleen Karr.
Snow Treasure by Marie McSwigan - based on a true story of heroic Norwegian children in WWII.
The Good Dog by Avi.
And on his bookshelf waiting to be read:
The Legend of Sleepy Hollow
Seven Day Magic and The Well-Wishers by Edward Eager
Charlie and the Chocolate Factory
Mossflower - a prequel to Redwall
My advice to keep them reading: they should always have a "book going", even if they skip a day or two reading. Expect reading alone from them the way you expect them to take their vitamins every day - it will become as necessary to them as it is to you if you treat it that way.