Horror being one of the few genres I rarely visit, “Pivot,” a claustrophobic gothic about a boy raised as an assassin by a mysterious cult leader, took nearly its full length to impress itself upon me. You need a certain mindset to sink into a book in which young Jack, fearful of father figure Cyrus, hones killing skills, at first on a perennially reincarnating uncle figure and then on strangers. L. C. Barlow writes convincingly, if a little at a distance, of this unlikeable world, especially the gory parts. But it is only when teenage Jack begins to explore his house for the source of Cyrus’s power, that the book accelerates and I thoroughly enjoyed the final third with its sinuous plot twists. For horror fans and toe dippers like myself.