Unholy Storm is one of the series of successful novels featuring Derrick Storm written by Richard Castle in the years before he met Beckett. Although the text novel is referenced in the show, unlike some other Storm books, it has never existed in the real world. However, it is one of the four Storm novels that have been published by Marvel as a graphic novel in the real world.
(From richardcastle.net; the Marvel graphic novel summary on Amazon is almost identical): Derrick Storm is back and he has a new, deadly case to solve. When the daughters of four high-powered international businessmen are discovered dead in New York, the NYPD scrambles to bring the murderer to justice. But when a fifth girl is found mutilated in a pool of her own blood, her prestigious French family hires Derrick Storm to run his own investigation and find the real killer. Storm has only one lead - a strange symbol drawn in blood. Storm enlists the help of the beautiful and daring Clara Strike, his CIA handler. Together they uncover a deep web of deception under the guise of mysticism and devotion.
References in Episodes
Always Buy Retail, (episode 1.6) – Castle mentions it while standing over the 1st murder victim, and reads some of it to Beckett at the loft
Veritas, (episode 6.22) – Castle can be seen pre-signing copies of Wild Storm and Unholy Storm in his hotel room while he calls Beckett. There are also large display posters of the book covers in his room.