User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-15860919-20130922185429/@comment-4555714-20130923221046

"Rick said he might do a third Camp Half-Blood during a Q&A last year (around September I believe) but of course it won't be out until at least 2015." -Kissmyfurryhind-quarters

"Q: @SB_and_WG will you be writing another Percy related series once HoO is done?  A: It's WAY too early to talk about that! Dunno yet."

Source: http://rickriordan.blogspot.com/2012_09_01_archive.html

Doesn't sound very promising....