5. 0-0 is definitely not a book move.
I don't understand how this is a Book Move.

There's probably some random GM who, while in an attempt to break into the elite, did a bit of study on that opening and as a result, it now has a name and qualifies as a book opening.

Book move means in the library of the engine - nothing else. Whatever you enter into the book of the engine is a book move.

Book move means in the library of the engine - nothing else. Whatever you enter into the book of the engine is a book move.
Not sure I agree with this since there were "book moves" long before there were engines ! Opening books that are in engines come from books, normally and without them the engines are not as strong as they are with them, ofcourse.

Well Afaf, yes, those opening moves didn't make a lot of sense at all. On the other hand, you guys (after all) played them...and seemed to think yourselves that they were stupid...would you mind explaining how any of this is supposed to make sense to the rest of us?
I didn't say we thought they were stupid.
I mean...my moves made sense to me. I sent the analysis to the Chess.com computer thing and when I was looking through it, I couldn't believe that my opponent's moves 4 and 5 were being called "book moves" by the computer despite resulting in the loss of a pawn. Understand?

I don't know...maybe I need to start getting human game analysis instead? When I see "book move" I expect solid trustworthy moves.

It says Sicilian Defense, Delayed Alapin variation. I don't know much about it, but I am sure you will find something about this variation while studying sicilian defense.
There are 3650 games in database starts with 1.e4 c5 2.Nf3 d6 3.c3 Nf6 line. I don't think all those games were played randomly.

Reb,
There is nothing to agree to. I as I understand the question was why the starting moves were not analysed and why it was called book moves. It is just how the library used by the engine is programmed. There is nothing to agree or disagree.
Opening books in engines do not come from books but from programmers. They may be influenced by books but mostly GM practice. There are thousands of books for engines. Often they are tuned to fit a particular engine.
There are also bugs or mistakes if not properly managed. 5. 0-0 seems to be a bad line put in the library for some obscure reason.

My connection is too slow to play with the database on this site right now. It is White's moves 4 and 5 that are odd to me. Are they in the database also?
I've seen the first 3 moves plenty because I don't respond to the Smith Morra thing.

I don't know...maybe I need to start getting human game analysis instead? When I see "book move" I expect solid trustworthy moves.
Who/what told you 5 o-o is book move ?

so you're saying you got the chess.com analysis done on this game and it doesn't give any of the moves as inaccuracies. Further it gives the first 5 moves for both sides including 5 0-0 etc as "book move"...very strange, I can't find 5 0-0 in any of my dbases.
Here is the chess.com explorer results for those moves and move 5

There is nothing to agree to. I as I understand the question was why the starting moves were not analysed and why it was called book moves. It is just how the library used by the engine is programmed. There is nothing to agree or disagree.
Opening books in engines do not come from books but from programmers. They may be influenced by books but mostly GM practice. There are thousands of books for engines. Often they are tuned to fit a particular engine.
There are also bugs or mistakes if not properly managed. 5. 0-0 seems to be a bad line put in the library for some obscure reason.
What is the hard part to understand?

There are also bugs or mistakes if not properly managed. 5. 0-0 seems to be a bad line put in the library for some obscure reason.
I think it's the lingo that creates the confusion here. As I understand when Afaf says 5th ply move, she means 5th half move, which is move 3. c3
Thats where the book moves ends. Move five castling is not registered as a book move in the database either.

@hermes3
she says
I mean...my moves made sense to me. I sent the analysis to the Chess.com computer thing and when I was looking through it, I couldn't believe that my opponent's moves 4 and 5 were being called "book moves" by the computer despite resulting in the loss of a pawn. Understand?
meaning unless I'm completely thick that white's moves losing a pawn were classed as "book" by chess.com analysis computer

@hermes3
she says
I mean...my moves made sense to me. I sent the analysis to the Chess.com computer thing and when I was looking through it, I couldn't believe that my opponent's moves 4 and 5 were being called "book moves" by the computer despite resulting in the loss of a pawn. Understand?
meaning unless I'm completely thick that white's moves losing a pawn were classed as "book" by chess.com analysis computer
I believe she is talking about half moves. If you read her original post, she says "first 12 ply moves" . But there are 7 moves posted. So she counts the first 6 move as 12 ply moves, and the "ply move 5" she mentions is move 3.
Moves, categorized as book moves ends with move 3. c3
EDIT: Since she stated otherwise in the following posts, seems like it is me who misunderstood her. Please disregard this post.

The first non-book move is 5 0-0 , 4 Be2 is ok because you cannot take the pawn on e4 yet due to 5 Qa4+ but you threaten it when you play 4... Nc6.

Opening books in engines do not come from books but from programmers. They may be influenced by books but mostly GM practice. There are thousands of books for engines. Often they are tuned to fit a particular engine.
I believe an opening book chess engines uses, and a book move are two different terms. Book moves are the moves that are in the Encyclopedia of Chess Openings. A software programmer can not decide what is a book move and what is not. On the other hand, a programmer can create an opening book (a computer file) for a chess engine and place any lines he wants into it.
I just got a game analysis for one of my games and I'm a little curious as to how the analysis does not identify any bad plays in the first 12 ply moves. And to top it off calls the first odd 5 ply "Book Moves".
Any input? It's easily possible that my opponent and I just can't see the brilliant continuation that could occur...