If you're looking to learn more about the Diablo universe, there are a number of different fan sites, wikis, and databases that you can browse for information about everything from crafting to characters and background stories for all your favorite NPCs.
But if you want to delve deeper into the history of Sanctuary -- and don't mind curling up with a good book (or three) -- you can also check out The Sin War trilogy, written by best-selling author Richard A. Knaak.
Set three thousand years before the fall of Tristram, the trilogy chronicles the journeys of Uldyssian-ul-Diomed, a simple farmer who would eventually become the first descendant of the Nephalem -- the children of the angels and demons who once inhabited Sanctuary -- to manifest the unspeakable power of that lineage.
Sought by both the Burning Hells and the archangel Inarius, Uldyssian must find a way to harness his power and overthrow the forces that would threaten humanity, setting into motion events that would not only reveal the existence of Sanctuary to the High Heavens, but shape the fate of mankind for all eternity.
(Pretty cool, huh?)
All three Sin War books are currently available on the Blizzard Store, as well as at many local retailers.