The term 'make book' is a phrase used in gambling, referring to the practice of taking bets on the outcome of an event. This phrase can be broken down into four crossword solutions: GAMBLE (6 letters), SPECULATE (9 letters), WAGER (5 letters), and BET (3 letters). GAMBLE and WAGER are synonyms for betting, while SPECULATE implies making a guess or prediction about the outcome of an event. BET is the most direct way of expressing the idea of making a WAGER on the outcome of an event.