 Book building is essentially a process used by companies raising capital through public offerings, both initial public offers (IPOs) or follow-on public offers (FPOs), to aid price and demand discovery.It is a mechanism where, during the period for which the book for the offer is open, the bids are collected from investors at various prices, which are within the price band specified by the issuer.The process is directed towards both the institutional as well as the retail investors.