It's a gripping little tale, very unlike the Marple television episode with Geraldine McEwan (my favourite Miss Marple). But that's how it goes when books become film. Many of the witty lines were intact, often said by different characters, and Marple herself doesn't appear til near the end of the book. Mostly the book focuses on Jerry, and is told from his point-of-view, as he convalescents in Lymstock, a small and rather inocuous little country town - or so he thought.
Poison pen letters, suicide, murders - smoke screens, as Jerry puts it. But he doesn't quite put it all together until Marple comes along and asks those famous, or infamous, questions that make him think. Marple is an interesting character. I hold her in esteem. She just doesn't trust people and expects the worst, even though she's quite kind to them. Marple is a watcher. She watches humanity and knows the faults will out themselves because they always do. People are people and they do bad things for stupid reasons like greed, hate, envy, lust. Her devout cyncicism must be difficult to eternally endure, but she does so with such aplomb and a skein or three of wool.
My hat is off to Miss Marple. I love her character. If I was ever in trouble I'd want her in my corner.
So the book - brilliant. Very twisty. If I hadn't watched the episode I wouldn't have known the murderer. There were a lot of very undesirable characters in the novel. People you knew could be the one, but a conspiracy on that scale would be ludicrous/impossible/cluedo-esque!
Read Christie's books. There's a reason why she's one of the greatest authors of the 20th century. She's fun and relaxing. Her books wrap you up and keep you in their little world until you're ready to face the real one. The best part, in her books the petty sneaky evil people are punished and truth triumphs. A sad discrepancy from real life where the opposite is more prominent.
You can read odd bits about the book in this Wiki.