The demigod diaries and demigod files have some short stories that add to the storylines of PJO and HoO, like how Percy met Bob and erased his memory and how Luke and Thalia met Annabeth and Thalia got her shield, so I would recommend those. I’ve read 3 chapters of the first book of Kane Chronicles, and so far it’s pretty good, but since I haven’t read the full series I can’t say. Magnus Chase is pretty good, I think it’s about as good as HoO.