... I hesitate to say, "of 2009," because of course you read things that weren't published this year, and how much richer I find writing about reading (Nick Hornby's Believer columns spring to mind) when a whole and full reading life is shared rather than just mock Printz or some pub-date obsessed cult of the ARC. That being said, I have read more recent things than probably ever before. These are some of my favorites:
Liar by Justine Larbalestier (2009) For almost two hundred pages, I thought I was reading this rather gritty realistic piece of fiction, and then it shifted to something else entirely. Really fresh.
Vast Fields of Ordinary by Nick Burd (2009) This book capture suburbia better than any I can remember, and the bittersweetness of the romance only adds to the overall melancholy tone. Perfect teen angst.
Skeleton Creek by Patrick Carman (2009) I know a few readers didn't like the genre-shifting, but I loved the way the video integrated with the text. An entirely new experience.
Geektastic: stories from the nerd herd by Holly Black and Cecil Castelucci (2009) Obligatory short story pick. Black and Castelucci are only two of the all-star YA stable penning pieces of ever variety of geek-dom, from theater geeks to comic-cons to academic teams. Interspersed with comics, too.
Some older books
The Difference Engine by William Gibson and Bruce Sterling (1991) I can't believe this book existed for so long without me knowing about it. It is the past and the future, simultaneously. I predict I can hand-sell it to a dozen boys post-Guy Ritchie incarnation of Sherlock Holmes.
Flashforward by Robert J. Sawyer (1999) I found a reference to the book when watching an episode of television series online. Sawyer's novel is set among particle physicists working on the large hadron collider at CERN, and the flashforward effect described is entirely different, it's great fun.
Tomorrow When the War Began by John Marsden (1995) Marsden's seven-book series charts the revolt of rather ordinary Australian teens after their country is invaded. The action sequences are fun, and Marsden's characters and their attachments original.
Life As We Knew It by Susan Beth Pfeffer (2006) In this novel and its companion, The Dead and the Gone, the moon is knocked much closer to the earth's orbit, disrupting tides, electricity, transportation, and blocking out the sun. Grim but enthralling. A third one comes out in 2010.
Frankly, I Expect No Less
Her Fearful Symmetry by Audrey Niffenegger (2009) When I try to tell someone what anything by Niffenegger is about, I start babbling like an idiot. It is about many things, none of which sound like they go together but somehow do is this amazing, dazzling, creative mass.
Juliet, Naked by Nick Hornby (2009) A book about fandom, but also love, loss, and aging. The first few pages depressed me profoundly enough for me to assent to the break-up of the fictional long-term relationship, proof if needed that Hornby's powers of persuasion are considerable. I would buy anything Hornby writes, he hasn't disappointed me yet.
Class by Jane Beaton (2008) A just-slightly-updated look at boarding school life, told through an endearing ensemble cast with students and teachers all keeping their own secrets.
Bad Housekeeping by Sue Limb (1995) Some naughty stories by the author of the Girl, 15 books. Like a smaller press, more intellectual version of I Don't Know How She Does It.
The Bride of Lowther Fell: A romance by Margaret Forster (1980) My knowledge of Forster was limited to Lady's Maid before this year, when I read most of her stuff. The memoirs are especially good, but this tale -- a modern and intellectual young woman moving her truculent nephew into the Lake district among menacing neighbors -- is an effective thriller.
London: the biography by Peter Ackroyd (2000) Obligatory nonfiction pick, more than nine hundred pages on the Kindle for iPhone, mostly read in the city. My favorite chapter was the one about the evolution of the tube, which listed the now-closed stops your hurtle past underground.