The notebook contains one message marked as important among a heap of spam:
"Set up a camp near the waste processing station and everyone who is going to be participating in this job will join you. Don't expect Black, because he and his people will be at Jupiter on a different mission. The army has suspended all flights into the Zone, so there won't be any trouble from that side. When stalkers find a way to Pripyat we'll send a guide and wait for the client's people there - they'll show us where the lab is.
Jackal."
― In-game description
The Mercenary notebook is a quest item in S.T.A.L.K.E.R.: Call of Pripyat.
Description[]
This item is found in the Waste Processing Station in Zaton, guarded by a large group of hostile Mercenaries led by Hook and Ridge. Owl provides a mission, Mercenary Camp, that requires the player to obtain this notebook. The player can either gun down all the mercenaries guarding the area or sneak inside the building and retrieve the laptop, which can be found in the upper level of the station, on a table. Alternatively, the player can wait until the mission Pripyat 1 is completed and retrieve it from the body of a mercenary in the area, as Hook and Ridge's team will have deserted the station.
Giving the notebook to Owl is necessary to start the mission One Shot later on (alongside completing the side mission Kopachy).