Janet Varney apologized for the misinformation about Book 2 release and it's definitely not coming out in April. Here is the link to her podcast episode56 and she talks about Korra in the beginning of the episode.
She also has a tweet warning about misinformation...
Misinformation alert! Korra premiere has NOT been annc'd. Lots of rumors/miscommunication-let's wait forofficial news from @<a href="https://twitter.com/nickelodeontv">nickelodeontv</a>— janet varney (@janetvarney) <a href="https://twitter.com/janetvarney/status/317484683829059584">March 29, 2013</a>
I got so excited about April and I'm glad to know that we still should expect something great!
https://twitter.com/Aprilstewart/status/319231163816361984">April 2, 2013</a>