Lore Book

A Lore Book in Deckard Cain's house.

Lore Books are in-game items that deliver background lore and/or story information. Lore books can be found in the world, and are triggered automatically by first encountering a new type of monster. All lore books are presented as the writings of an individual, often Deckard Cain or Abd al-Hazir. Other books are histories of an area, journals by NPCs, orders from Boss monsters, legends of the High Heavens, and many others.

All lore books can be listened to while the action continues, and can also be read in text form by accessing the in-game Journal.

Mechanics

Lore books are found in most areas of the game and describe that area, or add flavor text about it. First encounters with most types of monsters trigger a lore book as well, which can be listened to by clicking the icon on the lower right of the screen.

In-world lore books are only found if a particular character has not before "read" that particular book. Many areas of the game have multiple books, which are labeled with the number after the title, such as "The Fall of the Barbarians: Part One," a five book series found in the Keep in Act 3.

Deckard Cain's Journal being played in full audio.

Vasadan clarified the mechanics of lore books while grouped during the Diablo III beta[1]:

If you have lore already, it's going to disappear if you click on it. That way you do not get experience points for picking it up, but other people in your party still get the lore if they do not have it.

Achievements

Achievements are awarded for reading all the Lore books in a particular area, and/or for listening to all of the NPC conversations. These can often overlap; there are many installments to read in Leah's Journal through Acts 1-3, and these journal entries flesh out the ongoing story and supplement the actual conversations with Leah and the other NPCs.

There are many, many dozens of Lore Books found throughout the game, ranging from always there to quite rare.