The first book I read was Saffy's Angel by Hilary McKay.
The four Casson children, whose mother, Eve, is a fine-arts painter, have all been given the names of paint colors. Cadmium (Caddy), is the eldest; then Saffron (Saffy); Indigo, the only boy; and Rose, the youngest. When Saffy discovers quite by accident that she has been adopted, she is deeply upset, though the others assure her that it makes no difference at all. Saffy is the daughter of Eve's twin sister, who lived in Siena, Italy, and died in a car crash. Grandad brought Saffy, as a very small child, back from Siena.Sarah is the "energetic new friend". She has a lot of trouble walking due to a childhood illness and therefore uses a wheelchair. I feel like McKay does a great job of bringing Sarah to life without making it seem like she has anything to "overcome". You don't ignore Sarah's disability, but you don't ever find any spots in the narrative where you'd pat yourself on the back over your pity and understanding. She's just a fun kid who has a distinctive trait like just about everyone does.
At Grandad's death he leaves something to each of the children. To Saffy, it is "her angel," although no one knows its identity. How Saffy discovers what her angel is, with the help of an energetic new friend, lies at the heart of this enchanting story. Unforgettable characters come alive in often deeply humorous and always absorbing events to be treasured for a long, long time.
It's a nice, quick read that I thoroughly enjoyed at the age of twenty-nine. I'm not sure if it would have resonated with me as an older teen, but I know I would have been all over it anytime before I turned fifteen. This seems appropriate since Saffy and Sarah are about thirteen during most of the story. McKay has several other Casson family books that apparently involve Sarah and I can't wait to read them.
P.S. Also, it's set in England so you get to have a fun accent in your head while you read it. Unless you already have an English accent in which case you'll just have to settle for liking the story.