Books and writings are as common in Sanctuary as in any other world. The writings of men, angel and demon kind alike numbers in the thousands. A few of these are known works from the Diablo Universe for us mere earthlings.
A lot, but not all, of the material that sums up what we know of the lore about Mr D comes from “in-world” books, and as such is prone to fault and inconsistencies, like sources might be in our world. This is of course also a perfect way for Blizzard to keep most of the material “real” in terms of retcons and “canon” facts.
With an ever growing library of ideas that together sum up the Diablo Universe, inconsistencies are bound to happen, and as such, retcons will happen. Less interesting facts like specific years or months will fall aside to more interesting ideas such as fitting a multitude of creative works together as a cohesive whole.
All of a sudden, King Leoric was no longer a Northern Lord and he no longer ruled for decades in Tristram. Instead the pious lord ruled for a year or two and come from the East instead. Several similarly minor changes have taken place, and much of it explained as just errors by the individual authors of the books, as the author was an in-world scribe with his own knowledge and own motivations. Most famously hinted on by Abd al-Hazir in his Goatman entry, blaming much of the old lore’s faults on corrupt Vizjerei mages wanting to improve their reputation after the horrendous Mage Clan Wars.
Most of these books have no known author and only comes with a few short paragraphs each. This of course makes sense from our real world point of view, as it would be a terrible waste of man hours to write a multitude of books that often overlap just to give some backstory to the game anyway.
The two primary sources of these books are from the Diablo I Manual, Diablo II Manual and Tomes. Tomes are ancient books spread i dark and dangerous places with surprisingly little useful information (considering they are each a whole book). Never the less, some pieces of information can only be found in Tomes.
A majority of these Book excerpts are collected in Diablo Wiki as a handy reference for anyone interested in lore. More books will soon be added, both from the Diablo I Manual as well as from actual Diablo novels and other references.
So, you got the intro peptalk, what are you waiting for? Go ahead and read some REAL Diablo literature, from people living* on Sanctuary!
* At the time of writing they were alive – most likely anyway…