User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-12300635-20130921224853/@comment-4555714-20130923223302

It would be extremely confusing and random for anyone who has not read TKC--and, especially, for anyone who hasn't read The Son of Sobek--if Carter just popped up in the fourth book of the Heroes of Olympus to save the day. I wouldn't be surprised if Percy made a small reference to Carter, which can be ignored by the reader; but, for Carter to play a major role in the story would be strange.