I believe it refers to the fact that most equipment manufacturers don't bill until after the equipment has shipped (30 day net). The book (entering orders into the books) to bill ratio looks at the how the sales are coming in vs. the orders going out the door. You would want that book to bill ratio to always be slightly greater than 1.0, I would think. A massive increase of book to bill would suggest that production capacity needs to be increased. A drastic decrease of book to bill indicates that your resume needs to be up to date.