I like everything by Eva Ibbotson and Island of the Aunts is one of my favourites. Good for all ages, especially 8-12 years.
review by Fantastic Fiction
When the kindly old aunts decide that they need help caring for creatures who live on their hidden island, they know that adults can't be trusted. What they need are a few special children who can keep a secret-a secret as big as a magical island. And what better way to get children who can keep really big secrets, than to kidnap them! (After all, some children just plain need to be kidnapped.) Don't miss this wildly inventive and funny read from master storyteller Eva Ibbotson.
"Readers will not be able to put [Island of the Aunts] down! A fine choice for fantasy lovers." (School Library Journal, starred review)
"Eva Ibbotson does magic, humor, and fantasy for ages 8 to 88+, and you'll wish her books were never-ending, so enchanting are her characters and fiendishly funny her plots!" (Book Sense)