1. The 1e DMG, especially the Artifacts section and its many hints of lost cvivilisations and ancient magics.
2. The World section of the 4e DMG, in particular the spread picture at the start of it with the adventurers looking into the distance. The 4e 'setting' art in general is very evocative, and a real breath of fresh air after 3e's dreary art style. I like the Worlds & Monsters art a lot. I was so glad somebody at 4e finally 'got' the importance of landscape/setting art. Maybe Peter Jackson was an influence there? 4e's focus on style & atmosphere in general is a huge step up from anything I've seen from the D&D brand since about 1983-84.