City of Masks (Stravaganza Series #1)

City of Masks (Stravaganza Series #1) by Mary Hoffman

While sick in bed with cancer, Lucien begins making journeys to a place in a parallel world that resembles Venice, Italy, and he becomes caught up in the political intrigues surrounding the Duchessa who rules the city. Books in this series: Book 1. Stravaganza : City of Masks -- Book 2. Stravaganza : City of Stars -- Book 3. Stravanga : City of Flowers -- Book 4. Stravaganza : City of Secrets.

Neverwhere

Neverwhere by Neil Gaiman

A man goes to the aid of a woman pursued by assassins and discovers an alternative City of London, a subterranean, medieval world populated by "people who fell through the cracks" from the real city above. A fantasy tale, replete with demons and wizards.

Dingo

Dingo by Charles de Lint
Seventeen-year-old Miguel Schreiber and a long-term enemy are drawn into a strange dream world when they fall in love with shapeshifting sisters from Australia--twins hiding from a cursed ancestor who can only be freed with the girls' cooperation.

The New Policeman

The New Policeman by Kate Thompson

Irish teenager JJ Liddy discovers that time is leaking from his world into Tir na nOg, the land of the fairies, and when he attempts to stop the leak he finds out a lot about his family history, the music that he loves, and a crime his great-grandfather may or may not have committed. Sequel: The Last of the High Kings.