The courier who passed his name and trade to the man later known as Book, and the absent father of Moll, whose abandonment of his daughter shapes her distrust of everyone but L'ak.
The fourth Cleveland Booker is spoken of throughout the fifth season of Star Trek: Discovery. The name is a title handed from one courier to a chosen successor, and he gave it to the young Kwejian who became the fifth holder.
Early in the season Book learns that the thief Moll, born Malinne Ravel, is his mentor's daughter and therefore the nearest thing to family he has left. He tries repeatedly to reach her on that basis.
Moll remembers a different man. In Mirrors she says that her father left to earn enough to move them to a safe world and never came back, leaving her to fend for herself. Book's affection for him and her bitterness are never reconciled.