What does "sold to the book" mean in an auction?

At the ANR auction in NYC, during the Jung sale, a few times the auctioneer said at the end of a lot that the coin was "sold to the book for $XXX". What does this mean?

 

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I believe it means that the high bid was placed before the live auction began.

 

It can also mean that the owner's reserve was not met. At larger auctions, if you set the reserve, you "buy" your own coin at that reserve price. The acution house charges you the percentage on your reserve price.