In the context of stocks and trading, an order book is:
...the list of orders (manual or electronic) that a
stock exchange uses to record the interest of buyers and sellers in a
particular financial instrument. A matching engine uses the book to
determine which orders can be fully or partially executed.
An order book is organized by price level. Market depth refers to the number of shares being bid on or offered at each price point.
Order book value is the dollar value of interest on both the buy side and the sell side. It is not the dollar value of executed transactions. A $1.3B order book is an estimate of value of stocks that have not yet traded but are anticipated to be traded.