A book move is a move that follows established opening theory, the recorded body of well studied opening lines.
Chess openings have been analyzed for centuries, and the accumulated knowledge is called the book: named lines, main moves and known evaluations, stored today in databases of millions of master games. While both players follow it, nobody is really inventing anything yet.
Game review tools label a move as book when the position still appears in their opening library. Once a player chooses a move outside it, the game has left book and both sides are thinking for themselves. Leaving book is not a mistake; every game must leave it somewhere.
For club players the lesson is to learn the ideas behind your book moves rather than long memorized sequences. A player who understands why the moves are played will outplay one who recites theory and then drifts as soon as the book runs out.
The position after your move still exists in the site's library of established theory. The label stops appearing as soon as one of you plays a move that master practice has not catalogued.
For most club players, knowing the first 6 to 10 moves of each opening you play, plus the typical plans, beats memorizing 20 move lines. Real depth matters only in sharp openings where precise move orders are forced.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.