Great effort has been taken to keep this writeup as spoiler-free as possible. Want to know who the murderer is? Go read the Wikipedia entry. Trust me. You do not want to have this one ruined for you if you're in the process of reading the book or planning to do so.
The Murder of Roger Ackroyd is a mystery novel by Dame Agatha Christie. Its ending was so controversial at the time of publication that it earned its author scads of criticism and nearly found her blacklisted from a the mystery writer circuit.
The novel was published in 1926 and remains a classic of the genre to this day. It has, to date, been adapted as a TV movie only once.
Cast of characters
Hercule Poirot
James Sheppard
Caroline Sheppard
Roger Ackroyd
Hector Blunt
Mrs. Cecil Ackroyd
Flora Ackroyd
Ralph Paton
Ursula Bourne
Geoffrey Raymond
Elizabeth Russell
Parker
Charles Kent
The extremely basic gist*
Very little happens in the British village of King's Abbott, so anything out of the ordinary is likely to get the entire town twisted into a knot.
One such incident occurs when a woman named Mrs. Ferrars dies in her sleep. The town's doctor, Dr. Sheppard, suspects suicide and says the cause of death was an overdose of a sleeping draught. Mrs. Ferrars's lover, Roger Ackroyd, had received a letter from someone who claimed to have been blackmailing her. He shows it to Dr. Sheppard, who has come to his house for dinner.
Roger Ackroyd is murdered that night. Suspicion is immediately cast on a variety of the villagers, many of whom are either extremely shady characters, in debt, or both.
After interviewing, interrogating and analyzing every aspect of the case, Poirot unveils his stunning -- and 100 per cent correct, as usual -- solution.
Reaction (both from readers and 'experts')
As mentioned, the book and its controversial ending caused a firestorm. Though the novel is one of Christie's most popular works, the fact that it has not received the full-scale screen treatment may be the reason why its surprise ending is not as widely known as that of, say, Psycho. The TV adaptation was not terribly well received, as an important aspect of the story (that relates to the ending being such a big deal) was altered substantially. As well, Christie was particularly cautious about allowing film adaptations of her works because she was unsatisfied with a number of those that had been made previously. This may be why a full-scale film version was never released. In any event, the novel's purity remains intact.
Readers, however, were far more enthusiastic, as is evidenced by the book's continuing popularity.
Recommendations
Read it. Even if you're not into this sort of thing, read it now. Having been raised on Agatha Christie and other mystery novels and movies, I thought I could pick out the surprise ending before finishing the book. Wrong. Rather than make the ending obvious, all the buzz surrounding the solution only wound up adding to the surprise.
If you're going to read it, lock yourself in a room with the novel, some tea and a plate of cookies. This is not something you'll be able to put down, and once you see how random these clues seem to be, you will want to finish this book.
Resources:
http://en.wikipedia.org/wiki/The_Murder_of_Roger_Ackroyd
http://www.agathachristie.com/victims/site/guides/book_guide.php?id=6 | 
