I think there are four sorts of these books: one that shows mixed families/characters without any sort of commentary (Buzz! or Hello, Lulu), one that has some sort of discussion about the character's background (Two Mrs Gibsons), ones that has some kind of cheering squad for multiracialness (Black, white, just right!), and ones that are about the parents' relationship rather than the child's perspective (How my parents learned to eat, for example). I hate most of the cheering squad variety because they are too campy!
Here are some I can think of off the top of my head:
Hello, Lulu by Caroline Uff - series of board books with girl of mixed (black/white) heritage
More More More said the baby by Vera Williams - one of the stories has a white grandmother and a mixed baby
Buzz! by Janet Wong - family gets ready in the morning
I Love Saturdays y domingos by Alma Flor Ada
Two Mrs Gibsons by Toyomi Igus - about a girl's Japanese immigrant mother and African American grandmother
A Child's Calendar by John Updike - family goes about the year
Maxwell's Mountain by Shari Becker - boy tackles "mountain" climbing
Hello, Goodbye Window - loving depiction of girl with her grandparents. Are they or aren't they a mixed family?
African Princess by Lyra Edmonds - girl (daughter of Carribbean immigrant mother and white father) is laughed at for saying she is an African princess; she goes on a journey to understand
Also some people like the rather "abstract" book Little Blue and Little Yellow by Leo Leonni, which is sort of about color blobs mixing. And The Rabbits' Wedding by Garth Williams is another "is this or isn't this" story, but again, it's about the adults not the kids.