A Servant's Journal is looted from a Crumpled Corpse in the Caldeum Bazaar, northern-most corner (past the Searing Sands Inn). This is only obtainable during A Royal Audience.
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 II > Judge of Character