I came across a really cool art book called Vermis recently. The background story is that it's a guidebook to an unreleased dungeon crawler from the 90s. I've only had a chance to look at the preview images so far and it has a wonderfully dark, Gothic esthetic to it. Lots of bone, flesh, and decay.
I've been considering running some dungeon crawls for my monthly one-shot group and Vermis looks great to use as inspiration.