The book is very slow-moving. The beginning, which I guess is supposed to drill firmly into the reader's mind how selfish Iris is, really doesn't seem to all that necessary--at least not in the length presented. And it takes a good half of the book to get to the real action--the disappearance of Miss Froy. There is good build-up of Iris's frustration and her feeling that perhaps she really is mad (as suggested by several passengers) She is far more on her own than in the movie--in the book, she appeals to a professor and his young friend, a linguist, but neither of them truly believe her the way Gilbert does in the film. The novel, as a whole, didn't hold me the way the movie did and it ended fairly abruptly. Hitchcock is a master and mixed just the right amount of humor with his suspense. He adds a few characters (such as Charters and Caldicott, the cricket enthusiasts) for color and tells a ripping good tale. He also adds a more detailed, action-packed ending to add a bit more excitement.
If you're going to do both--I would definitely suggest reading the book first. It's possible that a reader coming to the story in book-form first might have a greater appreciation for White's novel. I don't think reading the book first will dampen your enthusiasm for the film. Two and a half stars for the book. Four stars for the film.
Challenges: 150 Plus Reading Challenge, Book Bingo, Book to Movie Challenge, Ethel Lina White, Mount TBR Challenge, Off the Shelf, Outdo Yourself, Vintage Mystery Challenge, European Reading Challenge, Around the World [For European & Around the World Challenges--takes place on a train in the "Balkans" which covers parts of Bulgaria, Serbia, Turkey, etc. I have arbitrarily attributed the action to Bulgaria.]