Before your Prince of the Apocalypse adventure begins is important to know the history of the Elemental Planes.
Shannon Appelcline editor-in-chief of RPGnet and the author of Designers & Dragons wrote an article that delve into the world of the Elements.
You can find resource to deepen Elelmental Lore like Deities & Demigods (1980), Manual of the Planes (1987)
Elemental Adventures before Prince of the Apocalypse like Egg of the Phoenix (1987) and of course The Temple of Elemental Evil (1985)
Elemental Settings as Earth, Air, Fire, and Water (1993) from Dark Sun and Secrets of the Lamp (1993) from Al-Qadim and the neverforgotten Planescape Campaign Setting (1994)
Latest resources are 3.5 edition Manual of the Planes (2001) and The Plane Below: Secrets of the Elemental Chaos (2009)