The world of unknowable objects --- magical items that most people have no idea possess powers --- has been quiet for decades. But the three current members of a secret society have remained watchful, meeting every six months in the basement of a bookshop in London. They are pledged to protect their archive of magical items hidden away, safe from the outside world --- and the world safe from them. But when Frank Simpson, the longest-standing member of the Society of Unknowable Objects, hears of a new artifact coming to light in Hong Kong, he sends Magda Sparks to investigate. Within hours of arriving in Hong Kong, Magda is confronted by a professional killer who seems to know all about unknowable objects, specifically one that was stolen from him a decade before. Magda is forced to flee, using an artifact that not even the rest of the Society knows about.

Cassie Andrews works in a New York City bookshop and lives an unassuming, ordinary life. Until the day one of her favorite customers --- a lonely yet charming old man --- dies right in front of her. She always loved his stories, and now she has nothing to remember him by. Nothing but the last book he was reading. But this is no ordinary book. It is the Book of Doors. Inscribed with enigmatic words and mysterious drawings, it promises Cassie that any door is every door. You just need to know how to open them. Then she’s approached by a gaunt stranger who calls himself Drummond Fox. He’s a librarian who keeps watch over a unique set of rare volumes. The tome now in Cassie’s possession is not the only book with great power, but it is the one most coveted by those who collect them. Now Cassie is being hunted by those few who know of the Special Books.