Many great novels have become movies or TV-shows, with varying results. This list will celebrate those that have become great TV-series as well. I strongly recommend reading the books first, not only because they are the original stories, but because they generally contain more detailed information and if you've seen the TV-series already it might be slightly boring going over all of it again. Enjoy!
Agatha Christie's Poirot
My absolute favourite detective masterfully played by David Suchet who, in my mind, has become Poirot entirely. Most of Christie's novels about Poirot have now become independent episodes, along with a few of the short stories. The longer episodes (the ones based on novels) are, in my opinion, much better than the short ones. Almost all characters are wonderfully cast, and very few episodes have disappointed me in comparison to the books (and I've read them all...). I just hope that they finish all the novels and that David Suchet wants to stay on for them all.
Pride and Prejudice by Jane Austen
I have a very low tolerance for any adaptations of Jane Austen's novels, but the BBC 1995 mini-series is one I can actually stand and even say that I almost love. I miss only one scene, where Mr. Darcy slightly awkwardly tells Elisabeth to go see Mr. Bennet who has granted him permission to marry her. But I'm being picky, and as such I can tell you that if I can stand this adaptation then it is excellent. I can imagine no other reason for there being no newer version made when all the other Austen novels have recently been re-adapted, other than that this is already as good as it could possibly get.
Dexter by Jeff Lindsay
My favourite serial killer, Dexter Morgan, whom I first got to know on TV a few years ago, is actually based on a character in a novel by Jeff Lindsay. The first season is almost identical to the first novel, but the second season has nothing what so ever to do with the second novel. I'm still trying to make time to read and watch the third. The TV-series is excellent in its own right, but I like both the extra background information that is given in the novel, and also that Dexter is slightly - if possible - less emotional in it. Also, since the plot is not at all the same I don't become so annoyed when they change little things, and can even accept that I don't always like the actors chosen for some of the parts.
Inspector Morse by Colin Dexter
Colin Dexter's novels were adapted into a TV-series and have recently become a spin-off with the character of DI Lewis (DS, as he was during the Morse episodes) which is absolutely excellent (not the least due to Lawrence Fox in the role of DS Hathaway) and actually is what earned this spot on the list. I'm not always very fond of Colin Dexter's novels, and while the Inspector Morse episodes are really good, they are slightly outdated for my personal taste. I might be stretching it a bit in adding this to the list but any excuse to tell you about Inspector Lewis, which is a fine an example of British crime TV as any, is worth taking.
Sense and Sensibility by Jane Austen
Yes, another Jane Austen novel - and here I claim to be so picky in accepting them - but I feel that since the latest, 2008, version is so close to perfection that I cannot make a list as this without mentioning it. Granted that the 1995 movie also is excellent, but I feel that a story this good requires a little more time than a movie can give. There are a few things I don't like about it, but it's just minor details that I'm sure someone less obsessive about Austen than me doesn't mind. Nonetheless, it is well worth seeing.
Extra:
Pippi Longstocking by Astrid Lindgren
Unfortunately, I'm afraid this is a Swedish adaptation for TV, which is why I added this as an extra item, but I simply couldn't leave it out. This was my absolute favourite when I was a child; I have always loved Pippi and her gumption. I'm sure it's available with English subtitles somewhere. I also have to warn you that it was made in -69 so the special effects are... questionable. But it is totally worth it, if not only for being an excellent time capsule of Sweden in the sixties, but also because Pippi is possibly the best character ever written.