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.