I was reading a beat-up paperback library book with my daughter over the Christmas break -- an entry in the "Babysitter's Little Sister" series -- and was surprised to find a really nice treatment of different families in general and adoption specifically. The book is "Karen's Twin," and although Amazon.com lists the paperback version as out-of-print, it's probably lurking in more school libraries than just ours.
The main character is proud to be a "two-two," with two of everything in the two households she lives in -- one she and her brother share with their mother and stepfather, and the other with their father, stepmother, stepsiblings, and a little sister just adopted from Vietnam. Part of the plot centers on paperwork problems surrounding the adoption, and I liked the fact that, while Karen sometimes thinks of the new family member as a pest, when she fears that the adoption may not go through, she springs to the tot's defense. Not to worry: the book ends with a "Happy Adoption Day" party.
A good deal of the story centers around an in-class "family tree" assignment -- the bugaboo of so many adoptive parents -- that is handled creatively by all the children; Karen, for example, makes a tree for each of her "families." When the class gets to invite members of their family for a special day, Karen invites her father and adopted sister, another brings the grandparents who are raising her, and another brings his foster parents.
The main plot -- in which one of Karen's classmates wants desperately to be her twin sister, and essentially stalks her for much of the book -- is interesting only in that the twin wannabe picks Karen because she's got the most interesting family in the class. As part of the "Babysitter's Club" sub-genre of grade-school fiction, "Karen's Twin" ain't exactly art, but it's amiable enough, and I much enjoyed the very matter-of-fact way in which family differences were treated. That's the way I try to treat them with my kids, too. If you're looking for ways to validate that point of view with your own personal pre-teen, this is a book worth checking garage sales for.