I have seen some commentary that implies that Stockfish did not have access to an opening book in its much-reported match with AlphaZero, but I cannot find a conclusive citation, and my inability to do so has me wondering if maybe Stockfish did have an opening book after all.
GM Hikaru Nakamura...called the match "dishonest" and pointed out that Stockfish's methodology requires it to have an openings book for optimal performance. While he doesn't think the ultimate winner would have changed, Nakamura thought the size of the winning score would be mitigated.
or, from a StackExchange answer:
Stockfish...wasn't allowed to use an opening book which makes it consider many moves in shallow depths.
It's worth noting that even if Stockfish didn't have an opening book for the canonical 100 game match, page 6 of the paper shows that AlphaZero is dominant when you start from a variety of common openings. Nonetheless I wonder why they would handicap Stockfish this way.