The Wanderer, by Sharon Creech, hardcover (very good condition; slight nick at top of dust jacket)
 
While thirteen-year-old Sophie is thrilled to be sailing across the Atlantic to England with her three uncles and two cousins, her trip is marred by nightmares about a huge, threatening wave. Sophie's cousin, Cody is concerned about her because, aside from her nightmares, she says odd things. Although Sophie was adopted just three years ago, she talks about her adoptive parents as if she has lived with them all her life. Stranger still, she tells stories that she claims their grandfather Bompie told her — a grandfather whom Cody knows she's never met. When Cody tries to ask his father and uncles about her, they tell him that it should be Sophie's decision when or even whether to talk about her past. Cody is distracted from his curiosity when the boat hits a wild storm that lasts for days, culminating in a giant wave hitting the boat and endangering all of their lives. It's as if Sophie's nightmares have become reality, and everyone is relieved when the boat makes it safely to land.

When the family reaches England, they find that Bompie is very sick, and Sophie comforts him by reminding him of events from his youth. It turns out that Sophie has been corresponding with Bompie since she was adopted and the stories she had told on the boat were not fantasies. She does, however, seem to mix up details of her life with Bompie's. While listening to Sophie talk to Bompie, Cody is able to piece together what really happened in Sophie's past. Both cousins come to realize that through their journey everyone in the family has gained a better understanding of each other — and themselves.  2001 Newbery Honor Book.
 
Grades 6-12