“Imaginary Friend” is Stephen Chbosky’s first new novel in 20 years, and it comes as a complete surprise. Chbosky’s only other published work, 1999’s “The Perks of Being a Wallflower,” concerned a trio of self-styled misfits navigating the complexities of high school, adolescent angst, sexual confusion and assorted personal traumas. Over the years, the book has led a charmed life acquiring millions of readers and serving as the basis for a popular film adapted and directed by Chbosky. Those many readers now have something new – and unexpected – to contemplate. Weighing in at more than 700 pages, “Imaginary Friend” is an all-out, not-for-the-fainthearted horror novel, one of the most effective and ambitious of recent years. Who would have guessed?