The Strange Affair of Adelaide Harris (1979)

The story, set in 19th-century England, starts with the "borrowing" of a newborn baby to see if the legend about wolves rearing it as their offspring is really true. Things become complicated when the baby is found by a passerby, who begins to care for it. The baby's parents summon a private detective to investigate the disappearance and he is convinced it is a part of a much more sinister plot.