Not always since their can be multiple book moves, no not always
Question on game review
For instance if black gets out of book and I continue a main line with white I may be blundering. I see.
Book moves, for the book being used, count as Book moves for Review and aren't considered for other classifications.
Does the system (stockfish ig) consider a book move the best move?
no. I have multiple examples where that does not hold true. Also the system reevaluates between displaying the opening Review and once in Analysis. I spent first five months here trying to decipher it all. And when you can set the depth and various other settings in Analysis, that all can change as well.
Book moves have been studied and names for hundreds of years. How good they are and which player ends up better afterwards isn't as simple as naming the moves. I know that the little analysis bar (+1.2, -7.3) tells how good each player is, determined by current position, which is an outcome of all moves before. You can tell when you blundered, though.
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Does the system (stockfish ig) consider a book move the best move?