Remembering Planescape
During the Nineties the publishers of Dungeons & Dragons released a couple dozen campaign settings. These settings varied from fantasy versions of ancient China and the Aztecs to post-apocalyptic wastelands, but none of them was more interesting than Planescape. The Planescape setting used D&D's cosmology of astral and elemental planes as its background for adventuring.
As a young DM I could not imagine how you could set a campaign on the Elemental Plane of Fire or in the Abyss without all players dying instantly. The answer is of course magic. Magical protection against the elements, magical breathing, magical communication, magical transportation.
The Planewalker's Handbook is a player's handbook for the setting written in an informal style that makes use of the invented slang of the Planescape world. The most interesting part of the handbook for me was the Factions. 15 Factions vie for power within the Planescape world and each one is based on philosophical ideas. That are factions based on solipsism, nihilism, objectivism, and many others.
All the Planescape books are out of print as of this writing. I came across my copy at a used book store. Maybe Wizards of the Coast will bring it back updated or some day, but until then I can only recommend you seek out a copy online if you are interested in the history of role playing games or are intrigued by the possibility of using D&D's Manual of the Planes as a campaign setting.