When I looked over the 109 books I read in 2012, I couldn't come up with a tidy top ten. Instead, here are some books I especially enjoyed and would recommend to anyone who likes the genre in question. Note that few of them are 2012 releases. Except in a few cases, e.g. a new book by a favorite author in a series I love, I don't make it a priority to read books immediately after release.
Favorite Historical Romance (New)
My Fair Concubine, by Jeannie Lin (2012). My Fair Lady in Tang Dynasty China, and my favorite of Lin's books to date.
Favorite Historical Romance (Old But Now Available as an Ebook)
The Wives of Bowie Stone, by Maggie Osborne (1994). The hero is the most heroic and admirable bigamist you'll ever meet.
Favorite Contemporary Romance
Doukakis's Apprentice, by Sarah Morgan (2011). I'm not usually a Harlequin Presents reader--I'm just not into wildly rich, wildly alpha heroes outside of SF or history, and even then I want them to be extra-awesome, brave, honorable, and brainy--we're talking Aral Vorkosigan or the Duke of Wellington here. But I've enjoyed Morgan's medical romances, and this book came so highly recommended that I tried it anyway. And I'm glad I did, hence its placement on this list.
Favorite YA Romance/Debut Book
Catching Jordan, by Miranda Kenneally (2011). Just a well-written book all around, and one of the few sports-themed romances I've read where I came away convinced the author thoroughly knows and loves the sport in question.
Wildly Popular Book That Actually Didn't Disappoint Me
The Hunger Games, by Suzanne Collins (2008). No need to say more, since I figure y'all have already heard of this one...
Favorite Mystery Discovered Randomly When I Heard Its Author Interviewed on NPR
Bruno, Chief of Police, by Martin Walker (2009). Lovely, leisurely-paced mystery that will make you wish yourself in France.
Favorite New Entries in Long-Running Series
Captain Vorpatril's Alliance, by Lois McMaster Bujold (2012). A lovely science fiction romance, albeit one that I doubt would have the same impact for readers lacking a long history with the characters and their world. Those readers should go grab Shards of Honor or The Warrior's Apprentice and start building that history!
The Scottish Prisoner, by Diana Gabaldon (2012). I really appreciate how Gabaldon writes soldiers. Jamie Fraser and John Grey remind me of the officers in my family and the ones I meet in my historical research in a way military heroes in historical romances often do not.
Most Useful Psychology/Self-Help Book
The Willpower Instinct, by Kelly McGonigal (2011). Explains why it's so hard to change and ways you can make it easier.
Best Food for My Inner History Geek
Moscow 1812, by Adam Zamoyski (2004). Gripping tale of Napoleon's invasion and retreat.
Guest of Honor, by Deborah Davis (2012). Race relations 100 years ago viewed through the lens of Teddy Roosevelt and Booker T Washington.
1493, by Charles C. Mann (2011). A history of the Columbian exchange and how it altered the course of the world in the past 500 years.
The Worst Hard Time, by Timothy Egan (2006). If you watched Ken Burns' The Dust Bowl and want to learn more, go here.