Agatha H. and the Voice of the Castle is the third novel. It covers the material from volume VII through half of volume X of the graphic novel: Agatha Heterodyne and the Voice of the Castle, Agatha Heterodyne and the Chapel of Bones, Agatha Heterodyne and the Heirs of the Storm, and Agatha Heterodyne and the Guardian Muse (until ).
Before the novel's final title was announced, several ideas were put forth by the professors.
- Working title for the third novel:
- Agatha H and her Unchaperoned Dungeon of Evil 
- Tentative titles for the third novel:
- Agatha Heterodyne and Her Unstructurally Sound Lingerie 
- Agatha H and the Self–Slicing Salami 
- Agatha H and the Worrisome Steam Frock .
- Agatha H and the Ever–Tightening Corset