After much research, I can confidently state that I think Book 2: Spirits will most likely be premiering in April 2013. As I have seen, mostly on Twitter, the cast of Legend of Korra are trying to hint us fans that it will indeed release in April of 2013. Being that they cannot give out the exact release date, they are definitely responding to fan's tweets when Book 2 is questioned. Take a look at David Faustino's (voice of Mako) tweet which was posted back in mid-August.
This being his official, Twitter Verified account, I find this quite exciting. Not only that, but many more sources have included/incorporated "April 2013" when it came to Book 2. More news and information will be posted on this site!