If you like The Westing Game by Ellen Raskin

What Eric Knew : a Sebastian Barth Mystery / by James Howe.
After Eric moves away he sends cryptic notes to Sebastian and David, who attempt to make sense of them by investigating a mysterious death.

The Eyes of the Killer Robot / John Bellairs.
Thirteen-year-old Johnny Dixon is put in jeopardy when he and Professor Childermass try to find a robot made many years ago by an evil wizard.

The Witchmaster's Key / by Franklin W. Dixon.
The Hardy boys' investigation of a museum robbery in England arouses the anger of a coven of black witches.

Chasing Vermeer / by Blue Balliett.
When seemingly unrelated and strange events start to happen and a
precious Vermeer painting disappears, eleven-year-olds Petra and Calder combine their talents to solve an international art scandal.

The Last Treasure / Janet S. Anderson.
Thirteen-year-old Ellsworth leaves his father to visit the relatives he has never met and eventually joins forces with Jess, his distant cousin,to uncover family secrets and search for their ancestor's hidden treasure.