How do you complete a trilogy of literary post-apocalyptic vampire novels totaling about 2,000 pages, with dozens of characters and a narrative that sprawls across the U.S. and much of the South Pacific over the course of more than 1,000 years?

If you’re Justin Cronin, author of “The Passage” series, it takes an arsenal of legal pads and Sharpies, and planning—lots of planning—with little room for tangents.

“Other writers, they sit down, they say, ‘I let the characters talk to me.’ That sounds more like a séance to me than writing a book,” the author says. “I could never do that. I’d want to throw myself off a building.”

Read more: How ‘The Passage’ Stacks Up Against Other Ambitious Book Series

The trilogy, which concludes with the May 24 publication of “The City of Mirrors,” required skillful plotting to keep its many characters, settings and subplots from spiraling out of control. It’s a tale of a viral outbreak that spawns legions of vampires who wipe out nearly everyone in North America save for a few pockets of survivors. Much of the story is set 100 years after the cataclysm, mostly in what used to be the American West, as the heroes face off against the vampire swarms controlled by their leader, Zero.