Beneath London — James P. Blaylock

The collapse of the Victoria Embankment uncovers a passage to an unknown realm beneath the city. Langdon St. Ives sets out to explore it, not knowing that a brilliant and wealthy psychopathic murderer is working to keep the underworld's secrets hidden for reasons of his own. 

St. Ives and his stalwart friends investigate a string of ghastly crimes: the gruesome death of a witch, the kidnapping of a blind, psychic girl, and the grim horrors of a secret hospital where experiments in medical electricity and the development of human, vampiric fungi, serve the strange, murderous ends of perhaps St. Ives's most dangerous nemesis yet.