In searching Pathfinder products I noticed an adventure on the City of Brass and The Great Beyond - A Guide to the Multiverse.
What I can't tell about The Great Beyond is if it has any kind of treatment on the Elemental (Inner) Planes or if there are any other resources that deal with what is the "default" cosmology of the Pathfinder RPG?
The Great Beyond is *the* book about the cosmology of Golarion, and of course it contains information about the elemental planes.
You can find an overview in the Campaign setting - this is basically the short form of The Great Beyond - and special lore about the lower planes in The Book of the Damned Vol. 1-x.