User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-4902593-20130914145422/@comment-5593616-20130916193730

@Kronos300, to be fair, we haven't seen Nico much so far in the series. I mean:

TLH - Nico is only mentioned as looking for Percy.

SoN - We see Nico for the first time in HoO, and he's again the cool mysterious dude. We don't, however, see him long enough for character development.

MoA - Nico is kidnapped, so obviously we don't hear from him.

I think HoH will be Nico's "developing" book of HoO. He's not kidnapped anymore, is on the Argo II with what's left of the seven, and RR himself said Nico will play a big part this book. Plus, its called the House of Hades, so we would have figured out that Nico would be kinda important anyway. Isn't it weird that BotL was Nico's developing book in PJO, and HoH seems like it will be for HoO, and they're both the fourth books.