The Secret of Zoom (Henry Holt, middle-grade, 2009, 291pp) by Lynne Jonell (author of Emmy and the Incredible Shrinking Rat).
Ten-year old Christina lives a boring and lonely life in an old stone mansion on the edge of a hill. Looking down, she can see school children playing games she's never had a chance to join in on. Looking up, she can see the hills and forests that hide Loompski Labs, where her father works. It was in one of those labs that an explosion claimed her mother's life, and since then she's been kept Safe by her well-intentioned but distant father.
But one day an orphan boy named Taft snatches a few precious moments from his work as a trash collector to ask her a question, and her life changes.
"Have you found the tunnel yet?"
So Christina hunts for the tunnel that is supposed to lead from her home up toward the hills. And when Taft makes a daring escape from the mysterious compound full of orphans where he has lived for years, he seeks refuge with Christina, and the two begin a perilous journey of adventure and discovery below ground...
What sinister fate awaits the orphans, carted up into the hills inside garbage trucks by sinister Lenny Loompski? What is the explosive secret of Zoom, the strange, magical metal with which Lenny is obsessed? Can the two plucky children turn the Zoom to their own purposes in time to foil Lenny and save the orphans?
The Secret of Zoom is reminiscent of Joan Aiken's books-- it's an entertaining adventure with a plucky heroine, and has a plot that, while not exactly absurd, goes well beyond what is credible and common-sensical (as the cover art suggests). School Library Journal named it one of their best 100 books of 2009; I personally wouldn't go that far (perhaps because I think it has more kid appeal than grown-up reader appeal). But I can easily imagine this somewhat charmingly eccentric story captivating younger readers--it's a fast, relatively easy read that offers an interesting take on the familiar plot of parentless children bravely thwarting the bad guy.
You can read an excerpt of the book here.