Eleven

Sam is almost 11 when he discovers a locked box in the attic above his grandfather Mack's room, and a piece of paper that says he was kidnapped. There are lots of other words, but Sam has always had trouble reading. He's desperate to find out who he is, and if his beloved Mack is really his grandfather. At night he's haunted by dreams of a big castle and a terrifying escape on a boat. Who can he trust to help him read the documents that could unravel the mystery? Then he and the new girl, Caroline, are paired up to work on a school project, building a castle in Mack's woodworking shop.
Caroline loves to read, and she can help. But she's moving soon, and the two must hurry to discover the truth about Sam.