From Diablo Wiki
The Diablo III manual will not be released until the game itself. Blizzard has not said much about the manual, but with the increased emphasis on storytelling in Diablo III, most fans are expecting the game manual to contain a good amount of lore, story, history, and other additional details to enrich the game experience.
The full Diablo III manual will be posted in this wiki once it becomes available.
 Diablo 3 Manual Leaked Images
Four photos showing pages of the Diablo III manual, while still on the printing press, were leaked in early January, 2012. They don't spoil anything, as they're just previously-seen artwork of the characters, with some blurry text that seems to relate background details about the classes.
The four images can be seen below.
 Additional Diablo III Materials
Ancillary products such as the Book of Cain and the Sword of Justice comic book have added many additional details to the Diablo III story, and the game's Cinematics will be great story-telling tools as well, as revealed in sneak previews such as the Black Soulstone cinematic.
- See the Diablo 3 Merchandise article for a very-detailed listing of all available D3 merch.
| Books & Lore of the
|Tomes - Lore Books  In-game: People World/Quests Bestiary Diablo I & II Extra: Manuals/Other Novels History|
|Diablo I Tomes||Diablo II Tomes|
|Diablo I Manual||Diablo II Manual||D2: LoD Manual||Diablo III Manual||Other sources|
Abd al-Hazir (Writings)
|Diablo novels||The Sin War Trilogy||Diablo III novels||Diablo comics||Other books|