From the New York Times bestselling author and legendary storyteller Alan Moore, the eagerly anticipated first book in an enthralling new series about murder, madness, and magic in post-WWII London.

Dennis Knuckleyard is a hapless eighteen-year-old who works and lives in a second-hand bookstore in 1949 London. Aspiring writer though he is, his life feels quite uneventful. But one day his boss and landlord, Coffin Ada, sends him to retrieve some rare books from a strange and paranoid dealer. When he retrieves the books, he discovers that one of them, A London Walk by Rev. Thomas Hampole, does not exist: It is a fictitious book that appears in a real novel by another author. If both Hampole and the book are made up, how did it come to be physically in Dennis's hands? Coffin Ada tells him they come from the other London, the Great When, a version of the city that is beyond time, in which every aspect of its history from its origin to its demise is somehow made manifest. There epochs blend and realities and unrealities blur and concepts such as Crime and Poetry are incarnated as wondrous and terrible beings. And Coffin Ada tells Dennis, if he does not return the book to this other London, he will be killed.

So begins Dennis' adventure in Long London. To return the otherworldly book, he must dive deep into the city's occult underbelly, meeting an eccentric cast of sorcerers and gangsters, including Grace Shilling, a sex worker who agrees to help Dennis with the caveat that she will stab him if he makes any advances; Prince Monolulu, an infamous horse race tipster who claims to be an Abyssinian Prince; and Jack Spot, a ruthless mob boss looking to cement his status on top of the city's underworld. But upon entering The Great When, Dennis finds himself at the center of an explosive series of events, one that may have altered and endangered both Londons for good.

Mystical, magnificently written, and hilarious, The Great When is Moore's most imaginative work yet. It is the unforgettable introduction to the brilliant, staggering, consciousness-altering world of Long London.