From my experience with Chessmaster, Extreme Chess,
Different databases have different "books".
Yeah, true
But what I'm currently wondering right now is why Chess.com is removing book moves. It used to be a book move but now it isn't.
Noticed the same for the Kholmov Gambit which I play a lot, but even the first few moves (classed as Petrov's Defence) aren't book moves anymore.
Of course all the previous sequences mentioned in this thread are not listed in https://www.chess.com/awards/<user_name>/books, but there does seem some kind of change lately.
We have completely overhauled our book move database to align it with well-known opening theory. We've added known opening moves to the "book" classification and are removing moves that are bad. With this change, you'll get a more accurate sense of how well-versed you are in your favorite openings—and which openings you could study more.
https://www.chess.com/news/view/chesscom-launches-game-review-v2
That was posted in March.
In my last game, I played the Tennison Gambit against the Scandinavian (e4, d5, Nf3) every single time I had analysed this, it had been marked as a book move, as it is a book move. Its a well-known gambit with theory behind it.
Recently, I analysed a game where I played the Tennison Gambit, and my Nf3 was marked as a mistake!
Another example of this is the Fried Liver Attack. where Knight takes f7 is now a Brilliant Move.
I am extremely confused as to why this happens now. If it is a new change, It makes zero sense and I hate it.