Two books:
The first, by L Sprague De Camp, is titled "The High Crusade," and is the unlikely story of an alien vessel landing in a small village in England, just before the nobleman was preparing to take his soldiers to the Holy Land on crusade. (I won't reveal any more details than that, but I highly recommend the book to anyone interested in science fiction adventure with an odd twist). It was, and remains to this day, one of my all time favorite non-series books.
The second, "The Wolfen," whose author, sadly, I have forgotten, is about a species of canines that has evolved over the ages, parallel to man, resulting in what became the source of werewolf legends. They are predators, with humans their natural prey, feeding on the vagabonds, drifters, and homeless, whose absence is rarely, if ever, missed. One kill goes wrong, resulting in an only partially devoured corpse, and a team of homicide detectives investigate the murder. Once more, I won't reveal any further details, but, to me at least, the story is absolutely bone chilling.
My disappointment came in both cases by the books being made into movies. "The High Crusade," instead of the adventure that the author depicted, was handled like a Monty Python comedy, and substandard even in that form. "The Wolfen" became simply a story about a pack of wolves living in an abandoned house in a slum in Chicago. The only similarity to the original story was the shared title.
And one Movie:
Edgar Rice Burroughs' "A Princess Of Mars." being made into the movie, "John Carter." My disappointment isn't with the treatment of the movis as taken from the book, I really like the movie. My disappointment is how the audience is staying away in droves, almost definitely destroying any hope of sequels.