User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7742465-20130623203319/@comment-4430596-20130623204042

In a sense, it's both. While both series are about the same "world" and have some of the same characters, they are two distinct series. The introduction of Camp Jupiter and the Roman gods solidifies this. Also, they have two distinct plot arcs. However, since HoO does somewhat continue the plot of PJO, you could consider HoH to be both the 4th book of HoO and the 9th book as well.