Module:
AtlasLootClassic/AtlasLootClassic_DungeonsAndRaids/data.lua

Category:
Scarlet Monastery Graveyard (line 1168)

Loottable:
Headless Horseman

Items:
Tome of Thomas Thomson

Additional context
According to wowwiki and live Wowhead this item is provided as part of a quest after Headless Horseman is dead by an NPC nearby called Sir Thomas and doesn't seem to drop off the body itself.

Sir Thomas is the Headless Horseman. At least he was the Headless Horseman before he went insane.
After you vanquish the Headless Horseman during the Hallows End event, the ghost of Sir Thomas will appear.

So i guess we can leave it as it is

Edit:
hm...according to wowhead the quest associated with this item was added with patch 3.3.0.
Its hard to say if this will drop or not.

Many tbc databases have this as a drop. It may also be just a quest item as you take "Free at Last!".
Few more days and we'll learn. Hallow's End starts probably 18/10.