Bookhunter is a "job" which can only be learned and done in Bookholm.
Description
Bookhiunters in the first volume of the series are mercenaries, who make money by bringing rare and valuable books from the catacombs to the surface.
To protect themselves from each other, dangerous books, wild animals, and other threats in the catacombs, they wear armour and helmets made from the bones and shells of the animals encountered underground. Many bookhunters kill and eat each other, and any other organism they encounter.
The only known exception to this latter habit, is Colophonius Regenschein; and its principal exemplar is probably Rongkong Koma: the author of a fundamental text on Bookhunting, and Regenschein's lifelong enemy. Both Regenschein and Koma die in the first book; and most of the Bookhiunters are killed.
Current state
In modern Bookholm, bookhiunters are replaced by Biblionauts based on Regenschein's work. These too wear armor and carry weapons, but do not kill innocents or compete among themselves, and treat residents of the surface with respect.