The Vandemark Mummy — Cynthia Voigt

A mummy has gone missing! It's not a terribly valuable mummy, but its disappearance spells big trouble for Phineas and Althea's father, who is the curator of a small collection of Egyptian antiquities bequeathed to the college where he works.

A whispered phone call leads Phineas and Althea to the vandalized mummy without revealing the identity of the thief. Then someone far more important to Phineas than a fifteen-hundred-year-old mummy vanishes. In spite of what the police think, he knows his sister would not have run away, and he knows she's in grave danger. But can he unravel the mystery in time to save her life?