Dead Girls by Abigail Tarttelin

When her best friend Billie is found murdered, eleven-year-old Thera – fearless and forthright – considers it her duty to find the killer.

Aided by an Ouija board, Billie’s ghost, and the spirits of four other dead girls, she’s determined to succeed. The trouble with Thera, though, is that she doesn’t always know when to stop – and sometimes there’s a fine line between doing the right thing and doing something very, very bad indeed.