From the back of the book:
Explore Infinite Worlds
The home of the gods. The essence of matter. The realm of demons. The birthplace of souls, and the cities of Hell. All these things and more await in the planes beyond Golarion. Brave mortals leave the cradle of their homeworld and cross the misty ethereal sea or the silver void to discover strange dimensions—some hauntingly familiar, others inherently deadly, and many alien beyond imagining.
Bargain with djinn over land rights ceded to the mephit king while fighting off roving patrols of the queen of the fire elementals. Sign treaties with the umbral dragons of Shadow Absalom. Join the archon armies on a sortie into the Abyss, or assist a cadre of devils guarding the winding river of souls through the Astral Plane. Invade your enemy’s dream realm, study your own past, or negotiate with a cannibalistic sentient demiplane.
This book describes all of the major planes of the Inner and Outer Spheres, as well as numerous demiplanes and lesser-known dimensions. It also provides maps of the nine planes of the Outer Sphere, and unleashes five new monsters unique to Golarion’s cosmology—soul-eating astradaemons, law-forging axiomites, trickster-fey that lurk in light, quasi-noble keketar proteans, and foxbard vulpinal agathions.
Looking for more planar adventure? Check out Pathfinder Module J5: Beyond the Vault of Souls, where the heroes must retrieve stolen soul-gems to prevent the sudden destruction of the multiverse!