Fall/Winter 2013 is my guess. Personally I don't think we will get any info on Book 2 until Comic-Con which is in late July.
The rest of July, August, and twhe first week of September will probably be dedicated to commercials and previews, and Korra Nation will start again.
I think episode 1 of Book 1 will air on the second Saturday of Autumn, September 14th, with the last episode airing December 14th. Just my 2 cents.
7.5 YEARS on the boards.