The term 'RULEBOOK EXPERT' can be broken down into three crossword solutions: REF (3 letters). A referee is an expert in the rules of a particular sport or game. They are responsible for ensuring that all players abide by the rules and regulations of the game. Referees also have the authority to make decisions on the field, such as calling fouls or awarding penalties. Referees are typically highly trained and experienced in their sport or game, and they must be able to make quick decisions in order to maintain the integrity of the game.