I have a shitload of books to read so this is going to take a while. Will update you as it goes.
It might end up looking something like this. Probably not in this order. (or nothing like this)
1 Grow Your Own Planet with the various areas labelled. Map left blank and tools and random tables given for gms and players to genreate a fiction tectonic history of their planet, should they wish to. So they can map out the general geography and get a clear idea of where they could go. Will leave the names the same as tribute to the scientists and geologists who discovered them but the meaning will change. So there should always be a Mohorovičić discontinuity. But Mohorovičić might be the explorer who discovered the edge of hell, or the stellar dragon that curls within the earth, or the Liche that trapped the god that powers the magma flows.
2 corpse of a river in the body of the sea. Karst diagram, most likely entrances. How Karst works, what it's like.
3 life without light extremophile and fungal personalities, treasures, magic, politics. Cracked and ancient evolutionary paths. Ancient genetics from the dawn of life. Forgotten Phylum.
4 his body blocked the route failures, successes, famous deaths and survivals, wealth, insanity. Legends to inspire and drive the explorer.
5 the vampire court/and others low level adventure area/linked areas. Near the surface so some relation to surface ways. Last light from the sun reflected by ancient lenses through lava tubes to dim dawn-gloom cave. Only way to kill vampires in the underdark. Also run by vampires, justice for blood, last safe space before you reach the middle reaches.
6 the fungal underground railroad Either hunt or protect the weird/innocent fungal life forms for fun and profit. Also a beastary of fungal life.
7 the hive of glass extremophile knowledge storehouse. Panopticon of the Underdark. Dangerous, corrupting. Dungeon of smokey volcanic glass where you can see the whole thing from any point, but nothing is as it seems..
8 classic climbs table/diagram of classic spelunking problems for educational purposes. Like that one-page illustration from a D&D book somewhere that shows someone falling and shows the falling damage from each height as well. May also include a kind of table/guide showing the different levels of climbing ability shown by humans so far, fading slightly into the superhuman, so DM's can convert all the different ways climbing works.
9 Cave Crawl Rules – fuck knows how I'm going to do this. Probably DEX for small scale, CON for large. Also creating larger cave systems
10 the shape of the space small scale immediate/tactical cave generator.
11 whispers in the dark rules for oral recitation and direction-riddles. Sensory information?
12 black market item costs, diagram to help map changes in what costs what going up and down. Some materials raise in price as you go down and visa versa. One page diagram/illustration?
13 Eigengrau. the impossiblity of darkness the eye can't see real darkness without light kinds of darkness, kinds of light, sight generally.
14 breakdown falling apart mentally and physically. Maybe also a reading list. Including Hallucination table never tell players if you are rolling on this or the encounter table.
15 nobles of the deep Random table for generating rivers, water features, water generally.
16 extremophile/fungal treasures and tools
17 people you meet and why they want to kill you npc's (is there such a thing underground?
18 maps and guides
19 encounters 2 part encounter table, geographical and animate
20 I search the body
21 dying in the dark what happens when the lights go out or (they search your body)
23 we happened to meet..
24 larger cave system equivalent to street system.
Still need – equipment/light breakdowns