Yeah, so Hollow Earth Expedition is pretty cool. System’s fast but flexible and the setting is terrific pulpy playground. I gotta recommend Secrets of the Surface World too. Tons of material from secret societies to weird science, more archetypes and skills and talents and resources and flaws and goddamn just more of everything. I’d say it’s “the rest of the core book” but it’s not like the core book is lacking. Well, it was lacking “beyond human” scale stuff, but that’s covered in the psychic powers, magic rituals, and gadgetry in SotSW.

What I’m saying is: the core book is good on its own, but adding Secrets of the Surface World to it is enough of an improvement that you’d be stupid not to go for both. That’s like 400 pages total, and the rules themselves take up very little space, so most of that is material for you to have fun with.