A market order to buy all shares available (the book) of a specific security from one specialist and all other broker dealers at current market prices. Buy the book orders are typically made by institutional investors

In that sense, “book” is a physical record of trading positions maintained by brokers and specialists before computers were introduced and took over the task of keeping an electronic record. Therefore, buying the book means that an investor is interested to have a long position in all shares available with a broker. This may drive up the price of a given security because buying the book implies it is in a very high demand.