The Finding — Nina Bawden

Alex is different from every other boy. He was Found eleven years ago in the arms of the giant Sphinx in the City of London. Ever since his Finding, Alex has lived happily with his adopted family, and instead of his birthday he celebrates his Finding Day because no one knows Alex's birthday -- except one person.

When his grandmother's neighbor, Mrs. Angel, dies, Alex's world turns upside down. Can one mysterious person change his life forever?