The problem with reading a book after seeing a movie is constantly comparing the visual medium with the written. In the case of Psycho, the movie is so ingrained in the pop culture consciousness generally, and my mind specifically, it was impossible to read Bloch's psychological mystery fresh. And that's a shame, because it's a very good book with a nice twist. Hitchcock isn't known for faithful adaptations of source material but with Psycho, he stayed true to the book. That's probably because it's a damn near perfect psychological horror novel so well-written I can easily imagine the shock readers felt in 1959, when Psycho was published, when they discovered who the killer really was. As perfect as the movie is, as much as I loved the shock of the movie twist, I'm disappointed I didn't read the book first. For me, there is nothing better than a book that takes me by surprise. Fifty-five years on, with the movie as what people think of when you say "Psycho," and with the shower scene an iconic horror movie moment, there was no way to capture the happy astonishment you feel when a well-crafted story takes an unexpected turn.
Still, Psycho is well worth reading. The characters have a depth not shown in the movie and if you're a writer, you're bound to learn something reading Bloch's tight prose.