The Black Rock Ledger pages only drop from a particular event in Act 2, in the area right past Khasim Outpost:
The Black Rock Ledger pages can be found [...] in the Stinging Winds desert in an old pirate ship that is stuck within a pile of sharp, black rocks.
The linked wiki page has extensive information, including screenshots of the potential event locations.
Every time I've found a page, there's been a man digging near the ship who remarks that as long as we don't disturb the ship, everything will be fine. He's then quickly killed by an ambush of enemies.
Every time I've encountered this event, I've found a Black Rock Ledger page.