Well suprizingly, its all in the manual for the games, and you can get the diablo books also, which arnt related to the main story, but take place in the diablo world, trust me its fun reading, in one of the books you have a drink at Atmas and you travel to Lut Gohlein. Read these books (first one is my favorite)
Diablo:Legacy of Blood
Diablo: Moon of the Spider
Diablo: The Kingdom of Shadow
theres another one, but its by a different author
Diablo:Legacy of Blood
Diablo: Moon of the Spider
Diablo: The Kingdom of Shadow
theres another one, but its by a different author