Because I am lazy, here is a list of books that may be suitable for younger readers. In general, this means they are (1) clean (with regards to sex and profanity) and (2) comprehensible. I may update this list sporadically.
A few minutes later: Added Blackbringer and The Secret Country. I also bolded the author's names to make the list easier to scan through.
M. T. Anderson's The Astonishing Life of Octavian Nothing, Traitor to the Nation, Volume 1: The Pox Party. Contains slavery and serious themes, of course, but mixed with dry humour. Volume 2 is scheduled for October.
D. M. Cornish's Monster Blood Tattoo. Book titles are Foundling and Lamplighter, with a third forthcoming. Excellent world-building and linguistics. The world isn't as grim as the series title makes it sound, either. Be warned that the whole trilogy will need to be read to get all of Rossamünd's story
Pamela Dean's The Secret Country trilogy. Second and third books are The Hidden Land and The Whim of the Dragon, and all three are intended to be read together. Children playing a game about an imaginary country with their cousins find their way into it in reality. However, I've tried giving this to most of my siblings and they all rejected it because of the "thees" and "thous", so be warned. The Dubious Hills is set in the same world but about different characters, and is probably my favorite Dean book. It might be better for slightly older readers, however—I wouldn't feel bad about giving it to a teen.
Jessica Day George's Dragon Slippers. Comic fantasy, but she does in an excellent job of adding real tension during the latter half.
Shannon Hale's The Goose Girl. Retold fairy tale. Sequels so far (though they are labeled companion novels and focus on different characters, they occur sequentially in time) are Enna Burning and River Secrets. Hale has also written Princess Academy and The Book of a Thousand Days, both independent books.
Patrice Kindl's Owl in Love and The Woman in the Wall. Two humorous books, not directly connected. The second also has some serious themes.
Elizabeth Knox's Dreamhunter and Dreamquake. One book split into two. Contains some sex (not very explicit but there's no missing it) and also torturous dreams. Solid, strong characters and a great historical feel of the early 1900s, even though it's set on a continent that doesn't exist in our world. A sequel would be nice (there is one big loose end) but I haven't heard anything yet.
Laura Ruby's The Wall and the Wing. Wacky humour in a slightly off version of New York City where people fly (but not very well) and monkeys talk. Sequel: The Chaos King.
Brandon Sanderson's Alcatraz Versus the Evil Librarians. Comedy with an (annoying) narrator who makes a point of cliffhangers, cryptic foreshadowing and digressions in the middle of fight scenes. Sequels are forthcoming, starting with Alcatraz Versus the Scrivener's Bones, which has a great sound as a title. Like Monster Blood Tattoo, the whole trilogy is needed to tell the complete story, although the end of the first book is not much of a cliffhanger. To be honest I should point out that my younger brother found the foreshadowing less cryptic than I did and guessed some of the surprises. Sanderson's adult books are also quite clean, if quite a bit longer.
Delia Sherman's Changeling. Has an idiot moment* but otherwise well done fantasy in an even more off version of New York City.
* That's when you yell at the character "You know you shouldn't be doing this!" and throw the book across the room before skipping several pages to get past the painfully stupid part.
Laini Taylor's Faeries of Dreamdark: Blackbringer. Great adventure with some clever world-building behind it. I'm looking forward to forthcoming books.
Megan Whalen Turner's The Thief and sequels The Queen of Attolia and The King of Attolia. If you haven't heard of these you should have.
Elizabeth Wein's Arthurian cycle, beginning with The Winter Prince. Historical fiction about Arthur's children and grandchildren. Contains several instances of torture and mutilation. In fact, The Empty Kingdom is one of the tensest books I've ever read. Successive books are A Coalition of Lions, The Sunbird, The Lion Hunter and The Empty Kingdom. Note: The last two are really one book and should be read together.