At Dusk We Die is one of Castle's bestselling novels written in the years before he met Beckett at the start of the series. It is one of his 'standalone' books, not part of the Derrick Storm series. Although referenced in the show, it has never been made available in the real world.
(From richardcastle.net) Still residing in the same tiny Texas town where he grew up, Ben Meltzer’s life in a peaceful one. He runs the local drugstore and has a growing family with his high school sweetheart. So when the Satan’s Creed motorcycle gang drive into town, he hardly pays them any mind. But after the entire town is ravaged by the Creed in a single night, he and his family are forced to flee for their lives and pray for the dawn. Because the Satan’s Creed are no ordinary biker gang – they’re ravenous vampires come to feed. As Ben attempts to keep his family safe, he quickly realizes that he must fight back – or watch as his family becomes fodder for nightmares come to life.
References in Episodes
Flowers For Your Grave (episode 1.1) – Book Ryan is holding when he announces “from the library of Katherine Beckett”
A Rose for Everafter (episode 2.12) - on Castle's book shelf.