Huw said:
Unless you object to reading children's/young adults' books, I'd add:
Weirdstone of Brisingamen by Alan Garner. He's done others, but that's his best
Artemis Fowl series: Artemis Fowl, The Arctic Incident, The Eternity Code, The Opal Deception by Eoin Colfer, plus some spin-offs.
I'd like to point out that Garner wrote a sequel to
Weirdstone, called
The Moon of Gomrath, which was much darker but just as good, if not better. I always thought there should be a third book in the series, but there never was.
Plus, there's now a fifth Artemis Fowl book:
The Lost Tribe. A bit more mature than the others, and by far the best in the series so far.
For other entertaining children's/young adults books, try The Squire's Tales by Gerald Morris: retellings of the Arthurian legends from the point of view of the knights' squires, damsels and younger brothers.
I also enjoyed the Einarinn series by Juliet E. McKenna. I haven't read her other books yet, but I've heard they're good too.
For something completely different, and very confusing, I recommend the Scavenger trilogy by K.J. Parker, with the caveat that it's very badly edited. And I'm not just talking about typos here: the wrong names are used, or familial relationships confused, stuff like that. Like I said, it's a confusing series to start with, and with things like that added in it keeps you guessing right until the very end (someone mentions that his brother was killed, then later it turns out to be his cousing, so you're wondering whether he's hiding something). But it was an interesting read if you like your fantasy dark and gritty.