The Magic Half, by Annie Barrows (Bloomsbury, 2008, middle grade, 211 pp).
Miri and her family have moved into a big old house out in the country. At last, eleven-year old Miri is going to have a room of her own, a strange shaped little attic room, away from the rest of her family. With one set of twins (boys) a year older than her, and another set of twins (girls), five years younger, Miri feels like the odd one out--the unremarkable, lonely, middle.
But exploring her bedroom, she finds a mysterious piece of glass taped to the wall. When she looks through it, she finds herself back in time, in the 1930s, face to face with Molly, another little girl who is even more lonely and out of place in her family than Miri herself.
Molly is an unwanted orphaned, living in fear of her bullying cousin Horst. It's clear that Horst is a no-good thief, and every day the threat of his violent temper hangs over Molly's head. Miri must figure out the magic of time travel to save her new friend....
Miri is a lovely heroine--smart and imaginative. She's naturally very unhappy about being seemingly stuck in 1935, but appealingly, a more trivial thought crosses her mind: "Great," she muttered. "When I'm in my eighties, I'll find out what happens to Harry Potter" (page 57). Her story is told briskly enough to keep the reader's interest, while allowing for plenty of description that brings the place and people alive.
This is an excellent book for those who like mysterious old houses, hidden treasure, unhappy orphans, and happy endings! I enjoyed it as a grown-up, and would have loved it to pieces as a ten-year old.