Just about every fantasy campaign I've ever played in has involved a cave system or cavern at one time or another. And though I'm a fan of dungeons, I really like it when there's more of a natural element to them. The "Five Room Dungeon" technique I first learned about in Johnn Four's Roleplaying Tips newsletter offers a great way to structure a quick dungeon (whether naturally formed or hand-made). But sometimes it's tough to come up with natural formations or challenges to even fit in the 5-room format.
That's where Caves & Caverns comes in from Creighton Broadhurst and David Posener. This resource for Pathfinder campaigns would most likely work in just about any D&D/OSR-inspired system, but offers a ton of options to consider when building cave/cavern systems for your adventures. Though it's designed for the "Ebon Realm," it's really synonymous with areas like "The Underdark" in D&D-speak, but can really be applied to any world with accessible areas underground.
So what do you get in this 88 page tome? A wide variety of bits and pieces you can use in multiple contexts. Descriptions and details about different features and hazards of the underground world, along with a sample cavern that includes many of the features included. 60+ stat blocks for creatures that are CR 1 to 13. Nearly 30 ready-made encounters EL 4 to 12. Whether you use the different features described here, the NPC stats and the various encounters are great to have in your bag of tricks for the occasional improvised adventure scenario.
And Raging Swan does a great job of making things easy to find. Even though there's no index, there are multiple tables designed to get you to the page you want to go to... Random encounters? Check. Roll a d100 on an appropriate table (or pick randomly) and run with the encounter given. Designing a quick cavern for your next session? Roll d100 to see which features might add a bit of spice to the night. Of all the random tables however, it's the "Cave & Cavern Dressing" table that's my favorite. Little things that will drive players insane or give them hope that things will turn out all right... "A faded chalk arrow on the wall points in the opposite direction to the PCs' travel" or "Faintly carved into the cavern wall is the Undercommon word for danger. The last letter of the word is missing and a smudge of dried blood on the ground hints at the carver's fate..."
The rest of this review is at Game Knight Reviews here: http://www.gameknightreviews.com/2012/01/supplement-review-caves-caverns-pfrpg-by-creigh-
[4 of 5 Stars!]