Book

A ‘book’ has different meanings and uses in different contexts. It can be an electronic record of pending ‘buy’ and ‘sell’ orders for a stock, maintained by the exchange, broker or other participants.
It can simply mean a record of positions held by a trader. A trader might take different positions simultaneously and thus maintain a record of all such positions, long or short in a ‘book’, also known as ‘Trading book’. An updated book will give the trader a realistic picture with respect to risk and exposure in the market.
Large Institutional traders can maintain such a ‘book’ for their customers and also for keeping a check on trading activity, future opportunities and risk/return.
In accounting terms, ‘Book’ means an accounting ledger.
A book may refer to a client list or a record of clients for professionals like financial advisors, planners, insurance agents or other business managers.