The Mystery of the Silent Nightingale (Three Cousins Detective Club)

is #2 in the Three Cousins Detective Club series.
==Book Description==
In The Mystery of the Silent Nightingale, Sarah-Jane Cooper needs a special gift for Kelly, her lifelong babysitter and friend. When Sarah-Jane spots a silver locket in the antique store window, she decides it would be perfect for Kelly. But then the locket mysteriously appears on her babysitter's doorstep! Who gave Kelly the necklace? Sarah-Jane and her cousins jump in to uncover another thrilling mystery!
(Description from paperbackswap.com)