It's hard to talk about this book without descending (no pun intended) into hyperbole. This is the best RPG book I have ever read, hands down, and I've read a lot. This book is so dense and richly layered that it's hard to consume any one part of the book. You have to take it in in peices and then go back and look at the bits you already read with fresh eyes.
About half the book is a monster manual, but it's nothing like what you get from a popular magic-user that lives on the edge of the ocean. Each monster is a just that, a monster: a horrific, terrifying, and unique abberation. There are no large crocodiles. There are crocodiles that have a symbiotic relationship with an intelligent fungi that uses them as an emissary between civilizations. There are no golems. There is a creature that is patchwork of stone and clay who has been wandering the veins, replacing bits and pieces of themselves, for millenia, with a random table that gives you descriptions like the Nightmare Lord of the ancient Demi-Kaz, the City on the border of Dawn, who survived the Great Flood and learned her art from the gods, and now quests for her heart, lost in a box on a sunken ship in the pits of the deepest ocean so she can destroy it and die. After reading this, you'll never want to say "and you see an owlbear" again.
After that is a section on the cultures of the veins. Again, these are reskinned human cultures. These are completely alien people with alien thoughts. Each one is singular and unique. They will be a challenge to roleplay, but done well, could be amazing.
After that is sections on how to build the veins and run encounters in them. The sections on how to run food and light are well-written, and they strike a delicate balance between handwaving away an important aspect and turning your players into accountants at spreadsheets. The section on how to generate the veins though is absolute gold. It's incredibly easy to do, and it gives you caves like nothing I've seen in an rpg. These are realistic caves. These aren't a leisurely jaunt through 10 by 10 halls that lead to big rooms perfect for manuvering around animated skeletons. These are small, cramped places in the dark, where you can get lost and die without ever seeing an enemy. One description goes like this:
"They can expect to be enfolded by stone on all sides and must go in darkness or push a light ahead of them. Often the only way to get through is by deliberately relaxing the muscles so that the volume of the body becomes more liquid and pliable. If the user becomes afraid, they may tense up and become trapped."
If you are taking your players below ground, you need Veins of the Earth.
[5 of 5 Stars!]