Norse Mythology, on the way. Rick Riordan needs to speed up those books. Wonder if he'll ever do a crossover with one of his other series though, that'd be amusing.
If you're interested in mythology, I've read some pretty awesome creation myths that I really recommend: the Yoruba Tribe, the Hopi, and of course the Enuma Elish. Greek, Roman, Egyptian, and Norse mythology are popular for good reason, but there's heaps out there.
Norse mythos is so much fun. I'd share some but ...not sure if it fits in with this site's rating. Especially the story of Loki and the goat.
*Egyptian Did you know about the crossover short stories between PJO/HoO and TKC? The Son of Sobek, The Staff of Serapis, and coming soon, The Crown of Ptolemy. Sobek is decent; Serapis is a lot better.
♛ This. Also, high speculations that the new MC is Annabeth's lil bro and if it didn't high disappointment. But I'm sure it will still be a great series.