The Body in the Library, published in 1942, deals with two murders, the first being the young dancer Ruby Keene, found in the library at Gossington Hall and the second being the unfortunate girl guide Pamela Reeves. Both murders are correctly identified and solved by St. Mary Mead's spinster detective Miss Marple, who has the advantage over the police because she noticed the dress worn by the body in the library and being the wrong one and that and that the corpse had bitten her finger nails. I find that the novel itself to be one of Christie's easier and faster reads as it doesn't have quite so many digressions as most of her other novels. I think that the main problem with actually reading Agatha Christie's works instead of watching the adaptations is that by reading the texts, it becomes painfully obvious that she didn't like writing detective fiction and would rather have written melodramatic romances instead.
The novel itself is a parody of all of the cliches of the detective novel. Christie says in her forward that she decided to write a variation of the body found in the library, so long as the library was "highly orthodox and conventional" and that the body found there was "wildly improbable and highly sensational". There are also about as many detectives trying to solve the murders as there are suspects. It's also one of her funnier novels, in terms of how stereotypical the minor characters are.
There have been two adaptations of The Body in the Library, one by the BBC in 1984 with Joan Hickson and one by ITV in 2004.
I hate the recent adaptation for several reasons. Most importantly, they changed most of the plot, characters and the identity of the murderer(s). And also the new Miss Marple is all wrong. She's kind of creepy, far too modern and a bit of an airhead. And the actress who portrayed Ruby Keene is far too pretty and ladylike, since the victim was described in the book as being unattractive, heavily made up and a "common, vulgar gold digger", which is a funner character to learn about. Although it has better photography and costumes, it's just all wrong! The problem with the new adaptations of Miss Marple by ITV and with the adaptations of Poirot since 2002 is that they so drastically change the plots, context and identity of the murderers that it just makes them too painful to watch.
Now, the previous BBC adaptation with Joan Hickson is the one which you should watch. It gets the character of Miss Marple correct, being that she is written as being very prim, proper, logical and nosey. She is also dressed correctly in her tweeds and smart hats and is shown to be knitting. And her handbag is the correct one. The ITV Miss Marple carries a huge Mary Poppins carpet bag, something which Miss Marple or anyone of her generation and temperament would never have done. The BBC Miss Marple carries a large leather handbag, which she is always clutching in her white gloved hands. It's amazing how big a difference something so small will add to one's enjoyment of watching it. Although the costumes in this adaptations aren't as colourful as the newer one (which do have an Eighties vibe to them), they are much more realistic to the time period and I do prefer some of the sets, particularly the cottage of Basil Blake. Both adaptations and the book should be available from your local library.
Here is a clip from the beginning of the BBC adaptation (which is actually three 50 minute episodes instead of the 90 minute ITV version):