Cairn is a solid, wonderfully illustrated, concise adventure game in the mechanical style of Into The Odd.
It's 24 pages, with extremely clean and readable layout, great illustrations, and one of the best character sheets I've ever seen.
Cairn dedicates a really surprising number of its opening pages to gaming philosophy, and specifically to teaching readers the mindset in which it's meant to run the best. There are three straight pages that go from big picture approaches to specific actionable advice, and all of it's good.
If you've GM'd a bit and want to learn more about different approaches to running rpgs, you may want to pick this up on the strength of its opening alone.
Mechanically, Cairn is almost identical to Into The Odd. It's very quick to pick up, you have three damageable stats and a layer of HP in front of them, and rolls are extremely simple to resolve. Cairn does have tons of unique items, but a bigger divergence is probably in how it handles magic. Instead of just having infinitely spammable relics, you have relics with finite charges and you have spellbooks to handle the stuff you would normally spam. And when you do cast a spell, you add a Fatigue to your inventory, which is cool as heck. Fatigue-casting funnels magic back into inventory management and survival, and it balances the fighter and the wizard by linking them through a common system.
For players, there's Into The Odd's very solid character creation tables, plus the addition of spells and new materials. Most of the book is player-facing content.
For GMs, there's a helpful bestiary and a useful guide for converting monsters, but no sample adventure, and there isn't really a lot of setting information either.
Overall, Cairn is a great little system that has a slightly more medieval fantasy feel than Into The Odd's weird science milieu. Everything in Cairn is explained very well, and the inclusion of spells is neat, and it runs on an extremely stable engine to boot. So if you have any old modules kicking around, this is a great way to dust them off. And if you're thinking of dipping into some old-school adventure gaming, this is a fantastic starting point. Cairn *might* not be for you if the thing you're looking for is lots of mechanical complexity, but if you want to get rolling in a couple of minutes, and if you want to play almost any old-school adventure on a single system, Cairn should have a spot in your collection.