Just a quick little update. Over the past week, April Stewart, a long-time voice actor from multiple Nickelodeon series, has made several statements on Twitter. These statements hint at an April 2013 release for Book 2 of The Legend of Korra. Here's what she's had to say:
"Legend of Korra fans- I can't believe I have to keep my mouth shut till Next April! It will be worth the wait! I promise!" 
“@EleventyKeeyy: dudes i'm pretty stoked that @Aprilstewart has joined LOK voice cast #aprilcomesoonplease See you in Book2 #LegendofKorra" 
Both tweets from April seem pretty clear that Book 2 of LoK will be released in April 2013. The first episode of Book 1, Welcome to Republic City, was also released in April, so another release in April sounds pretty reasonable.