The account book is a quest item in Fallout 2.
This account book lists a series of monthly payments made to the mercenary band from the Bishop Family in New Reno. The payments depend heavily on how much "pressure" the mercenaries put on Vault City. When recovered, it contains payments for a three-month campaign of terror against Vault City, with options for a one-month extension if they are stubborn. It can be delivered to Lynette to prove the raider-New Reno connection.
Raider's cave: Inside the safe, unlocked with the raider captains' dog tags.