In economics, BTB is book-to-bill ratio. First you calculate the three-month averages of orders booked and sales billed. Then you divide orders by sales. This quotient reflects the relation of the potential revenue (orders) to actual revenue (sales). When orders increase faster than sales, the business is growing, so BTB gets higher. When orders decrease faster than sales, the business is in decline, because the demand (orders) decrease. BTB is quite accurate reflecting the condition of the market, but it fails when the market simply fluctuates, or when sales decrease faster than orders. BTB is not always so reliable: the BTB of the semiconductor market has been negative in 1999, when the industry was growing fast.
Source: Semiconductor Magazine. http://www.semi.org/web/wmagazine.nsf/5d1f03730e8999a6882565f0006108cc/85fd605ad415e15c88256872007a8d96!OpenDocument