Tome of Set Dungeons
From Diablo Wiki
The Tome of Set Dungeons is a lore book found every game in the Royal Quarters, a small non-random section of dungeon located up the stairway to the right once players enter Leoric's Manor. This area was introduced into the game in Patch 2.4. Prior to that the stairway to the right was blocked by rubble.
Players who are geared in one of the class Item Sets will receive a lore book that offers a clue to the location of their Set Dungeon when they click on the Tome of Set Dungeons. (Players may also find these clues via random lore book drops in other areas.)