Some 10 years ago Deborah Harkness, history professor at the University of Southern California, was perusing books at an airport shop. A display of fantastical werewolf and vampire novels, including the recently released fourth installment of the Twilight series, caught her eye. Harkness had recently completed writing a dense academic tome, spending three years holed up in libraries, so she was particularly riveted by this alternate world, a free-spirited departure from the 16th-century Elizabethan era that was her specialty.

“What if these creatures lived side by side with humans, but no one knew,” she mused. “They would have to function just like everyone else, holding jobs and going on dates. They would always be one car accident away from discovery.”

Contemplating this secretive duality led Harkness to write A Discovery of Witches, the first title in her bestselling All Souls trilogy—currently being adapted into a television series—that revolves around a reluctant witch, Diana Bishop, and a dynamic vampire geneticist, Matthew Clairmont. Having investigated such topics as female medical practice in early London and Queen Elizabeth I’s astrologer before her fiction debut, Harkness says that delving into such newfound macabre territory is “a natural extension of my life as a historian and scholar.”

Her most recent effort, which debuted this month, illuminates the intricacies woven throughout her trilogy. World of All Souls: The Complete Guide to a Discovery of Witches, Shadow of Night, and the Book of Life (Viking) is a detailed whimsical compendium, peppered with personal anecdotes, that Harkness says was born out of a passion for her readers and their questions. Featuring character bios, synopses, and maps, as well as recipes, architectural descriptions, and rich illustrations, the book offers an immersion into the alchemy and culture that shape the All Souls trilogy.

In her classroom, Harkness encourages active reading, and that’s exactly what World of Souls aims to do. Embracing a collaborative approach, she called upon a team “to help pull this project together. We wanted it to be playful and engaging, not a dry encyclopedic treatment. We included elements that aren’t in the books, [and] shared historic details and processes so readers could go on a voyage of discovery.”

Such nuanced exploration is also on display in the forthcoming Time’s Convert (Viking, Sept.), in which Harkness chronicles the life of Marcus Whitmore, the scientist, vampire, and son of Matthew Clairmont first introduced in the trilogy. “I started imagining the backstories of the other characters,” Harkness says, “and I had another book.”