A record of all the positions that a trader is holding. This record shows the total amount of long and short position that the trader has undertaken.

For example, a trader with a very simple "book" may hold two positions: one long position of XYZ stock worth $1,500 and a short position worth $2,500. Keeping an up-to-date book allows a trader to be aware of his or her positions, and would be a good step in preventing a trader from being exposed to any one position for a long period of time.