The Book of Heortling Mythology is an amazing bit of secondary world creation that reveals a mythology that is deep and many faceted. Stafford takes the same tactic that he used in King of Sartar of offering myths from lots of different viewpoints, though it's not as refined here (likely because this book was collated from the contents of several other works, rather than being created as an original piece). Nonetheless, it's an amazing work that depicts a mythological cycle that has the depth and verisimilitude of something from a real culture.
If you want HeroQuests, they're here. If you want to understand the foundations of Orlanthi society, it's here. If you just want a good, dense volume of new myths, it's here.