Sherlock, Lupin and Me The Dark Lady by Irene Adler*

Really written by Italian writer Alessandro Gatti
Translated by Chris Turner

Irene Adler spends a summer in a French town Saint-Malo where she meets William Sherlock Holmes and Arsene Lupin.  While playing at an abandoned manor, the threesome discover a dead body washed upon the shore.  They find a suicide note in the nameless gentleman's pocket

but soon surmise that someone had murdered him.  Then it is announced that someone stole Lady Martigny's diamond necklace.  Are the two crimes connected?
With determination, observation and wit, the three children solve the seaside town's mysteries before the police.
Clever and exciting.
Irene Adler is a fictional character in one of the Sherlock Holmes books.  This series has them being friends as children.
*Highly recommend for older readers.  See the warning/points to discuss below.
Warnings/points to discuss:  The following are mentioned but not elaborated upon: suicide and adultery. A man is murdered.
Guided Reading Level: T (4th grade but recommend for older readers because of content)