Bought as a decision to complete my 1st Ed AD&D hardback collection, I recall this and it's sister book being released in the late 80s. I never bought it as by then I was investing more on other RPGs, and I already had enough with the usual rulebooks and three monster manuals.
Douglas Niles is a good writer, and a prolific one too during that period. The book covers a lot of ground: rules for Underground environment such as climbing and swimming; new equipment; non-weapon proficiencies (then in their infancy); fatigue and exhaustion; combat rules underground; loads of stuff on design of underground settings; cultures and races of the 'Deepearth'; and a campaign environment, the Lands of Deepearth.
All of this was interesting and transposable to later editions without much effort. I found the Campaign considerations, essentially advice on design, gameplay, story structures, narrative technique very interesting. Much in here would have been fascinating at the time, although now with internet resource and large sections in the 5e PHB anD DMG, it seems more commonplace. But kudos to Doug Niles for bringing all of this into a mainstream hardback, rather than it just sitting in Dragon articles.
All over, a nice read. Look forward to the wilderness one.