Cons: if you've seen the movie, you know the story (including the twist ending)
When Mary Crane's boss hands her an envelope containing $40,000 to bring to the bank she thinks her troubles are over. She impulsively drives to her boyfriend's town, intending to use the cash to pay off his debts so they can finally get married. But heavy rain makes her miss a turn and she ends up spending the night at the Bates Motel.
If you've seen the Alfred Hitchcock movie you know the story. There are some differences, particularly with regards to Norman Bates, who in the book is 40, overweight, and converses more with his mother than he does in the film. Mary's also given more motivation for stealing the money. Mrs. Bates is quite horrifying, especially given how Norman covers for her.
It is a highly suspenseful book, with a great twist ending if you haven't seen the film (and even if you have, the skill with which the author makes Norman's home life bizarre but plausible is wonderful).
With such a great story it's no wonder Hitchcock thought it would make an excellent film, and kept the plot so close to that of the book. It's short (just over 200 pages) and a very quick read.