This waiting is so silly and it kills me! Firstly they said March 23, March 29, now it's April and we even don't know when it will start yet :-( i really hope it's this month, we can't wait that long for 14 episodes where one episode is about 20 minutes
I suspect the season will air this summer. Think about it Avatar's main audience are kids ages 8-12. So during the summer kids will be out of school and can watch Korra without worrying about getting to school on time.
Well as of now, Mike and Bryan are working on two books at once, the second and the third. Bryan posted about it on his Tumblr awhile back, so at least we know they're working hard. i think it's a good idea to not have the second book airing until some of the rest of the work is done, so they don't have to worry about any of it piling up in the future.
We just need to be patient because we all know this season is going to be epic, as it shows in all their hard work.