open a book on something—(Bookmakers) take bets on certain contests:
- There is a good deal of interest in the outcome of this ballot, so I thought it might be fun to open a book on the event.
an open book=—used of a person or thing that can be readily examined or understood:
- He kept to himself personally, but his military life was an open book.
See also: close the book on something / close the books on something.