'The Phantom of the Opera' by Gaston Leroux is a novel blending mystery, romance, and horror, set in the Paris Opera House. It tells the story of a disfigured musical genius, known as the Phantom, who haunts the opera house and becomes obsessed with a young soprano, Christine Daaé. Leroux, a French journalist and author, masterfully creates a suspenseful atmosphere that explores themes of beauty, love, and obsession.