Brian records and calculates the results of the chocolate sale in his account book. He's recording dollar amounts and number of boxes sold accurately. But, The Vigils tell him who gets credit for the sales. Often credit is given to students who sell little or nothing, like The Goober when he stops selling chocolates. In the real-world business, this would be considered fraud.
Like Obie's notebook, Brian's account book is a record of crimes. Of course, these crimes are harder to detect. Also, like Obie's notebook, it's a symbol of talent being perverted at Trinity. Brian is being pressured to use his time, talent, and energy for something which doesn't personally benefit him, and which doesn't benefit his fellow students. By the end of the novel, his actions become downright dishonest, when The Vigils force him to start cooking the books.
In the process, Brian gets caught up and begins to enjoy himself, and, perhaps, loses some of his personal dignity. And he knows it. After the sale ends, he plans to stay far away from everybody involved in the chocolate mess, but we think this will be harder than it might sound. The Vigils and Leon now see Brian as being valuable to their evil enterprises and easy to manipulate. We're guessing they probably won't let him go without a fight.