The Doom of the Haunted Opera — John Bellairs

Two kids find a haunted opera score capable of raising the dead!
 
When Lewis Barnavelt and Rose Rita Pottinger explore an abandoned theater, they discover an unpublished opera score entitled “The Day of Doom”. Ignoring a strange omen, they show it to their music teacher, who heralds “The Day of Doom” as a masterpiece. Little do they know that the eerie Henry Vanderhelm, the composer's grandson, arrives-with a plan…A plan to awaken the dead and enslave the world!

Books by John Bellairs:
 
The Doom of the Haunted Opera
The Figure In the Shadows
The Ghost in the Mirror
The House With a Clock In Its Walls
The Letter, the Witch, and the Ring
The Mansion in the Mist
The Specter From the Magician's Museum
The Treasure of Alpheus Winterborn
The Vengeance of the Witch-Finder