Crumbling Journal is a tome found in Act II of Diablo III. It is found on skeletal remains in the Desolate Sands. The remains don't look obvious, they don't stand out any more than any other sets of bones.
It is required for the Judge of Character achievement.
It reads as follows:
My mother told me not to join the mages' army. I should have listened. In the morning they're sending the infantry against the Vizjerei. I don't know how we're supposed to survive. Maybe we're not. It's so beautiful here. Maybe if I hid in the trees, they wouldn't notice me. — Mage Clan Soldier