What is Book Building?

Legal Definition
Book building is a systematic process of generating, capturing, and recording investor demand for shares during an initial public offering (IPO), or other securities during their issuance process, in order to support efficient price discovery. Usually, the issuer appoints a major investment bank to act as a major securities underwriter or bookrunner.

Book Building is an alternative method of making a public issue in which applications are accepted from large buyers such as financial institutions, corporations or high net-worth individual, almost on firm allotment basis, instead of asking them to apply in public offer.
-- Wikipedia
Legal Definition
Getting new investors to buy a new security issue. The more the interest the more demand on an issue and that changes the price of it.
Legal Definition
The process of gathering firm ORDERS or INDICATIONS OF INTEREST for a NEW ISSUE of securities. UNDERWRITERS involved in the new issue carry out bookbuilding, and the results are used to determine final deal pricing and order allocations.