From [OED][1] for the adjectival sense of "booked":

> 4.a. Of a seat, room, appointment, etc.: engaged or arranged in advance; reserved.

So I think it means the passenger who already has a reservation on the coach. "by this history" refers to the preceding recounting of the story; one of the passengers has a booking, the other two don't.

I didn't see anything in the earlier text that actually describes a passenger having a booking, so perhaps it's meant figuratively to refer to the passenger who was singled out in the preceding text.

  [1]: https://www.oed.com/dictionary/booked_adj?tab=meaning_and_use#16630522