Veins of the Earth puts all other "underdark" themed sourcebooks to shame.
I would recommend this book to anyone considering a game in a subterranean realm. Veins of the Earth provides the tools and inspiration to make such a place as alien and scary as it really should be, and to test your players and their characters with challenges they might not have considered before.
It's clear that the author either has experience in real-world caving, or else has done a great deal of research into caving. I've done it myself and the writing in this book quickly dredged up my experiences with uncanny precision. Letterboxes, sumps, crawlways, chimneys, flowstone... the absolute darkness. It's all here.
The crowning glories of Veins of the Earth are the Rapture; the cave generation and mapping system; light, lamps, and "lumes"; and the expanded climbing and exploration mechanics. I also really enjoyed the entry on the dErO. These alone are worth the price of admission.
If I were to offer any constructive criticism (I have a physical copy too - which is of astounding quality), I'd suggest settlements and quests. There is some information on a few cultures in the Veins, but information on living spaces is almost non-existent. I wouldn't expect a detailed account of an unlikely underground metropolis, but some guidelines on where the PCs can expect to buy food, light, equipment, hirelings, etc. (without being enslaved or otherwise betrayed) would be extremely valuable. The Veins are also presented as an extremely dangerous and uncomfortable place to venture. It is clearly the GM's function to come up with quests and macguffins for the PCs to look for underground, but a few suggestions would have been very welcome. I'm pretty sure my players would just say "nope" after a few days in the Veins unless there was something extremely important or valuable keeping them down there.
In conclusion, this is an excellent supplement and successfully captures the foreboding darkness and alien architecture of a massive underground space. GMs should buy it and read it, at least for the four topics I mentioned above. Your caves will never be the same again.
[5 of 5 Stars!]