It is the cleanest leading indicator in cyclical hardware and semiconductor businesses, often turning two or three quarters before reported revenue does. A reading that crosses 1 in either direction usually moves the whole sector.
Single quarters are noisy because large orders land unevenly, so use a rolling three-month or annual figure and compare it with the same point in previous cycles.
Example: Northwind Tools books $889M of orders against $840M of revenue, a 1.06 book-to-bill. The prior year was 0.94, consistent with the 18% backlog increase.
Related: backlog, revenue, guidance, sequential-growth, year-over-year