Start with high-confidence gothic & romance lit crit books, then narrow by subgenre or adjacent topics.
The Vampyre and Carmilla
John Polidori, Sheridan Le Fanu
Zofloya
Charlotte Dacre, Kim Ian Michasiw
Mazes of the Serpent
Roger B. Salomon
Gothic and Modernism
John Paul Riquelme
Reading at the Social Limit
Jonathan Elmer
THE GOTHIC TEXT
Marshall Brown
Tales of the Troubled Dead
Catherine Belsey
Ecogothic
Andrew Smith, William Hughes
Goethe's Concept of the Daemonic
Angus Nicholls
The Castle of Otranto
Esh Alladi, Horace Walpole
The Phantom of the Opera
Gaston Leroux
DRACULA
Bram Stoker