I just walked into the keep depths on Inferno and killed a handful of mobs right near the entrance. I noticed a guard corpse in the middle of the floor and clicked it, out popped a bloodstained letter. When I tried to pick it up, it just vanished and nothing happened. Looking at my achievements, it looks like I already have the bloodstained letter checked off. So either I just got it when the book vanished, or I had it from the last time I played through this area more than 8 months ago.
My character has been around since near the beginning of the game launch, so it doesn't seem like there is an issue with old characters. I played for a couple months, then took a really long break, then started playing again about two weeks ago.
Here is all the info about my current state when it dropped:
- Difficulty: Inferno
- Monster Power: Level 3
- Magic Find: 120% (No Nephalem Valor, I just logged in at the beginning of this quest)
- Objective: The Breached Keep - Find the Keep Depths Level 2
- Location: The Keep Depths Level 1 (I just entered from Bastion's Keep Stronghold)
- Looted from: "Dead Guardsman" (not completely sure on the name)
My guess is that it can drop anywhere in the keep depths, and that you have just been incredibly unlucky. I hope this information helps somehow.
By the way, the item text for the book is the same color as tradeskill items and gems when it is sitting on the ground (light blue).