From Diablo Wiki
The Torn Letter can be found in a Dusty Bag in the Caverns of Araneae below Wortham in Act I. You are not required to be on any particular quest in order for the book to appear although you will probably be on Trailing the Coven.
Only one of your characters needs to find the journal for you to be credited with it.
To find out which journals you have found so far go to your Achievements list (press Y) and check:
- Campaign > Act I > Historian of Tristram
|Found in the Caverns of Araneae in a Dusty Bag|
|Part||Text||Found on Quest|
|1||Chancellor Eamon, It is of utmost importance that we secure my manor from the traitorous rabble in Tristram. The caves to the east are too close to my land! They must be blocked - completely obstructed. I believe the archbishop Lazarus has magically bound some arachnids of late. They may serve us well... -- His Majesty, King Leoric||Any|
Torn Letter is in some way involved in the following achievement(s).
| 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|