I picked it up recently. It's not quite 1e, being designed for Adventures Dark and Deep, but it's all compatible anyways. I'd probably recommend using the standard Hyperborea stats for monsters when there's overlap, but that still leaves some fun creatures (I was glad to see stats for the winged things from The Festival). It was written as an adaptation to a D&D style so there's some stuff in there that might need some slight reflavouring for a pure Hyperborea game, like elven translations of the Necronomicon becoming Keltic or something. Some fun spells in there that'll fit right in too. I'm not quite sure how necessary the new classes are for Hyperborea, and same with the madness and skills rules, but if you're looking for a way to expand your game it's a very solid choice.

Kickstarted this and own the physical book. Great fodder for Hyperborea.