Le Guin's Earthsea series, if you haven't already read it. I didn't like the first ones so much, but they get better as they go on...

(It occurred to me that there may be spoilers in that wikipedia link, so be careful.)

I didn't get on with Jonathan Strange (the book) at all. Just went on and on and on without much interest.

I did like the Mortal Engines series a lot, which is another "kidult" series.

Also liked the Temeraire series: set in the Napoleonic war but with dragons.

It's a bit darker and more adult but Steph Swainston's Castle series, beginning The Year of Our War is superb.

Have you read Neil Gaiman's books? Some are gruesomely weird, but some are more intriguingly weird ... I'm just re-reading Neverwhere. It's fascinating - and very like the alternative universe of Lyra's Oxford.
If you like the slightly comedic, you could also try Terry Pratchett's Discworld series of books.

Seconding the Le Guin series. Very very awesome.

Steven Brust's Vladiad is pretty much the greatest series of books I've ever read. They are short and extremely fast-paced. There are 19 planned and 11 written. I loved the books so much that I paid $40+ for a tattered paperback copy of Yendi (I think it was?) because it was out of print and I was obsessed. Now I have read the entire series at least three times. Do yourself a favor and pick up Jhereg. You'll read it in one sitting.

If you're into hard sci-fi, Dan Simmons's Hyperion series is fascinating.

Want something a bit more adult and bit less literary? I'm on book 5 of the Sookie Sackhouse series - Aka True Blood. They're fantastic. These are not romance novels though they do have it in them. They're more mystery / sci fi.

I absolutely loved Good Omens and laughed my way through it's dogma. The husband swears by Pratchett's discworld series.

Have you picked up The Hitchhiker's Guide to the Galaxy series yet? It's brillian.

Most anything by Tom Robbins is among my favorites.

Heinlein is always great... though heavier reading.

*its not it's. pardon, not enough sleep to avoid being an idiot.

The best thing I've read recently was Bone. I don't know how it eluded me for so long but gods bless the public library for having it so enticingly on their shelves.

The Le Guin that I've most enjoyed was The Dispossessed, which definitely has some number of esses in it, though not necessarily the number I've included.

Two fantasies from Peter S. Beagle that I recommend highly: A Fine and Private Place and The Innkeeper's Song.

I have "Lyra's Oxford" - it's a little epilogue-y book that Pullman put out after the main series. Email if you want to borrow? But it's very very little.

I think you'd like Discworld too.

Seconding Earthsea books and Neil Gaiman.

Although it has been a long time since I read the His Dark Materials trilogy, I recently read The Children's Hospital by Chris Adrian, and was immediately reminded of them. The book is not fantastical, but it is definitely otherworldly, and faces off against some similar religious issues as posed in HDM. Plus, it gets bonus points for being long and just plain weird.

Not to derail too much, but what do the bunnies say about Garth Nix' Old Kingdom series? I've been meaning to read it, but haven't yet.

I have the first three Garth Nix books and I think they're excellent. The whole way necromancy and magic is presented is really fresh and the characters are delightful. (Disclaimer: total fan, called my cat Mogget, Facebooked author etc etc)

Another writer you may like if you're after light fantasy would be Robin McKinley. I suggest "Blue Sword" and "The Hero & The Crown" as a good place to start. She also did a retelling of the Beauty & the Beast fairytale (called "Beauty", unsurprisingly) and another interesting one called "DeerSkin". Word of warning: she's a bit horse oriented, so there will be sudden 12 year old moments from time to time.

You might like the Kushiel series by Jacqueline Carey if sex and bondage don't offend. The world it's set in is an alternate Earth and I think it works really well. The first three books are excellent. The next three, not bad but getting a bit tired.

I recommend the Seer King trilogy by Chris Bunch - refreshing for it's mostly angst-free main character. I detest whiners and Damastes might be smug and arrogant, but at least he shuts up and gets on with the job. Plenty of sex and violence.

I'd recommend more but I'm only half unpacked. Perhaps China Meiville might suit you? I'd avoid "King Rat" and start with "Perdido Street Station" or (my favourite) "The Scar".

I really liked Sabriel. The next two, not so much - not that they're bad or anything, just not up to the level of the first. I wanted to smack the main character (different than the first) around for being so self-absorbed and -indulgent.

Terry Pratchett! His Discworld stuff will keep you busy for a loooong time. It's fantasy/sci-fi and really funny and insightful to boot. If you want to read them in order, here's a good reading guide. Don't be frightened by the young adult novels - they're just as good (and sometimes better).

Try the d'Artagnan Romances of Alexandre Dumas.

ninazer0 and eatdonuts already posted my suggestions. I'm just jumping onto the "I hate it when a book series ends, too," validation train.

Caitlin R. Kiernan's "Silk" (International Horror Guild Award, Best First Novel 1998) and its sequel, "Murder Of Angels". These are dark fantasy with a liberal dose of existential horror with vivid characterizations and themes strong enough to manage such a mix. "Murder Of Angels" occasionally pushes into experimental territory and succeeds quite well at it.

If you have a taste for the world of Lovecraft, then you may want to get Kiernan's "Threshold" (International Horror Guild Award, Best Novel 2001), the first book in a series of five where the world becomes ever more Lovecraftian for reasons that would be spoilers. These books deal with ancient horrors and Kiernan is a paleontologist, so if it crawls out of the earth, she knows what she's talking about. These are the Dancy Flammarion set of books, although Dancy herself is a secondary character in "Threshold", and they compare well with books by Poppy Z. Brite.

None of Kiernan's books end with cliffhangers; they are each self-contained and can be read in any order. If her name sounds somewhat familiar, she was the primary writer for the The Dreamer, the spinoff of Neil Gaiman's Sandman. Kiernan's style is direct and uninhibited; if it smells like shit, Kiernan says it smells like shit rather than make up some metaphor.

Just jumping into the thread to leer generally and say "You can check me out anytime, Citrus!" *winks sleazily, hiccups, and goes on her way*

I'm reading Carlos Ruiz Zafon's new book. I'm liking it - but am not far in because I'm trying to pace myself. I LOVED his first book though.

Also - Susan Cooper! The Dark is Rising is AMAZING.

GORMENGHAST!

It was inspired by Peake's experiences growing up as a Brit in China and being one of the first westerners to see the Forbidden City (and later, one of the first to liberate Dachau.)

It's like the anti-Tolkien and is like a big takedown of fnatasy tropes. just the bestest. This is so up your alley.

WoOoOoOo suggestions! Thanks guys! I've got more than enough books to check out now. Feel free to suggest more, fantasy or otherwise.

::musters as salacious a wink as possible at Ms. Strange::
;)

Last and First Men, then Star Maker. Basically the entire future history of humanity (18 species!) and of the whole cosmos.

I re-read these recently.

The Years of Rice and Salt is excellent too. An alternate history of the world which takes as its jumping off point Europe wiped out by the Black Death, so that China and Islam become dominant civilisations. It follows the same set of characters through different reincarnations over many centuries.