If the Silver Age had a paradigmatic book, 1986's Dungeoneer's Survival Guide is a good candidate. Despite its name, the DSG wasn't really all that focused on "dungeons" in the traditional sense. Rather, its subject matter was subterranean locales, specifically caves and cave complexes -- the "wilderness" beneath the surface of the world. A better name would have been the Spelunker's Survival Guide, but I admit that it doesn't exactly roll off the tongue. The DSG is also notable for having first introduced the term "Underdark" to D&D, which has proven remarkably resilient, surviving both as an appellation and as a concept virtually unchanged through multiple editions of the game, a feat even the Outer Planes have not managed.
Dungeoneer's Survival Guide covers a lot of ground, offering up an overview of the many obstacles adventurers might encounter underground, along with rules for adjudicating them. So, we're treated to discussions of cave-ins, subterranean waterways (and flooding), poisonous gas, vulcanism, oxygen deprivation, and more. These discussions all go a long way toward bringing the Underdark to life as a physical environment rather than as merely a backdrop against which to battle the drow or mind flayers. The Underdark itself is thus the biggest "monster" of them all and the DSG does an excellent job of making all these details seem integral to the creation of a challenging subterranean adventure.
When this was released, I was entranced by it. I very quickly became an advocate of the DSG and attempted to employ it in my home campaign. Being a fan of Verne and Burroughs, the visions author Doug Niles conjured up of the Underdark were difficult to resist -- a perfect antidote for a young man tired of "boring old dungeons" and looking for new and exciting ways to challenge his players in subterranean locales. In practice, all the detail and advice Niles provided was easily forgotten, however, because it (mostly) got in the way. Nice as it was to know, for example, how long a character could hold his breath or leap across a chasm, never mind the effects of underground lava flows, it was just too much for me and proved quite dispensable in play.
I still adored the book, which was fun to read and quite inspirational, but very little of its contents ever found a permanent home in my game. The exception, of course, was non-weapon proficiencies, which eventually became an integral part of AD&D. Though originally introduced in Oriental Adventures, it was in the DSG (and its 1986 twin, the Wilderness Survival Guide) that concept really took hold. Again, at the time, I thought non-weapon proficiencies were brilliant and added much needed depth to characters. In retrospect, I can see that they were a well-intentioned mistake that laid the groundwork for many of the worst excesses of 2e. But I wasn't alone in my love for them, which is why, despite their optional status in the Zeb Cook-edited Players Handbook, I never met a single player or referee who did not use them and lean increasingly on them for adjudicating almost any non-combat action in the game.
Dungeoneer's Survival Guide is also notable for popularizing another indulgence of the Silver Age: isometric maps. The DSG promoted this style of "3-D" mapping as the best way to represent the complexities of subterranean environments. And, again, I won't deny that I was much impressed with them. These maps really were beautiful to behold and did seem to make it easier to design "realistic" underworlds, with multiple elevations and the like. As with much of the Silver Age, though, functionality was sacrificed in the name of esthetics. Nice as isometric maps are to look at, they're much less useful in actual play, not to mention much more time consuming to produce than traditional orthographic maps. Nevertheless, TSR regularly used isometric maps in their products during this period.
In the final analysis, Dungeoneer's Survival Guide was a brilliant failure and emblematic of many of the trends I eventually came to dislike about the direction of AD&D in the later years of TSR. It was -- and is, I'd still contend -- an inspirational book, one whose text often conveys the beauty and wonder and horror of subterranean locales so well. As a rulebook, though, it's less than ideal, simultaneously being too nitpicky and too easily cast aside. And its intention of adding new depths (no pun intended) to both the physical environment of adventures and to adventurers themselves was another nail in the coffin of the Old Ways. It's a pity, because, despite it all, I still have a fondness for this book, even though its publication was another step down a path whose ultimate destination I strongly dislike these days.