Definitely surprising for me, too. After Killzone, I didn't expect Guerrilla to so masterfully develop that plot and lore, but it's really, really great. I think their hiring of John Gonzales (Fallout: New Vegas) was huge for delivering an original world.
I would buy a Horizon book series in a second. Without spoiling, one thing Horizon does incredibly well is introduce and explain lore that's highly interesting and rooted in some interesting ideas... politics, culture, technology... It's a triumph that the story of the game is so satisfying and self-contained even while introducing all these ideas, but they could be explored so much more.