This book was about identical twin boys, one of whom was thought to have drowned as a baby. His brother, however was convinced his brother was still alive, and the book was about his search for him, which was eventually successful. His brother had been snatched by a distraught mother; her own boy had just drowned, and it was this child who was wrongly identified as the lost twin. I had thought the author of this book was James Hilton, but if it was (and I am quite probably wrong about that) I cannot identify the title.