I've been following Jesse Morgan's work on his Ronden Marr setting for a while now. Jesse's Ronden Marr setting is the eponymous dwarf city (probably the last city in the world) plus the Undercavern below it. I did a review earlier on the Ronden Marr Campaign Setting Player's Guide. Like the earlier book this one is also system neutral which I consider a big plus. I've never felt like I really grasped D&D's Underdark as a place to adventure and thus avoided it. This guide seemed like a potentially fresh approach and I wanted to see Jesse's take on the concept.
First off, a quick bit about the art. The first thing I do with any gaming product is jump all over the art. In this case Jesse did all the art himself, which I think is pretty cool. Here are a couple nice examples:
The intro with historical background leads you into the setting well. The content is presented as a compilation of information brought back from earlier expeditions, many of which were hideous disasters. There is a well-written section "A Feel for the Undercavern" which did indeed provide a good feel--and without resorting to a lot of system crunch you'd have to unpack mentally. The maps are great but don't come with a grid or scale. That's excellent for showing them to the players, especially since the earlier expeditions often weren't able to map carefully, but slightly inconvenient for the sort of DM who wants full details. A set of DM's versions at the back with square or hex grids would be a good addition for later. The locations are all pretty cool. Descriptions don't go into heavy detail but give you a clear idea of what's there--well, except for a couple weird mystery places such as the creepy Isle of Nyares. There's just enough detail to work with while staying away from specific system crunch. Also, all the major locations have their own map or illustration.
Jesse surprised me (in a good way) by including an environmental danger called "reality fissures": "Throughout The Undercavern, there are places where the fabric of reality breaks down. These fissures lead to other planes and demi-planes, or even into the magical fabric of reality itself." These allow for fun random old-school weirdness in a region where multiple "dungeons of the mad wizard" would make no sense.
The book includes several intelligent races/societies living down in the depths, unique creatures, and impressive sections on mushrooms and lichens which provide great information on dangers and benefits of the many types. The information on mushrooms and lichens isn't just for flavor: these items serve as gatherable resources (if the PCs can properly identify them) to keep parties supplied down in the middle of nowhere.
Finally, I loved the labeled diagram/drawing of Rock Formations (Appendix C), a fun visual catalog of underground geophysical features.
So if you want to see an approach to doing an Underdark this book is a good, usable guide. It's reasonably priced so grab a copy at DriveThruRPG.