Kazu has been responding to several tweets asking him when to expect Book 6 lately.
He has confirmed that Scholastic's plans are to release the book in Fall.
Kazu stated earlier in 2013 that he did not know what Scholastic's plans were, but that he did expect an autumn release. Today, it seems Scholastic has finally made a choice. Additionally, Kazu also confirmed on Twitter that he would start working on Book 7 immediately after he finished Book 6, so there will be no pause in between. This is likely due to the bacterial meningitis Kazu suffered earlier last year, hence the delay of Book 6.