Stephen King notes at the end of his newest book, “Billy Summers,” published this week by Scribner, that he wrote the novel between June 12, 2019 and July 3, 2020. Though it’s far from a “pandemic” book — it’s the latest addition in King’s later-career crime novel period — the sense of the walls closing in that is suffused throughout the book will likely feel familiar to those of us who spent months indoors over the past 18 months. “Billy Summers” tells the story of the titular character: a hitman with a heart of gold who takes what he hopes will be one last job.