Book move not from the start?

A chess opening refers to the initial moves of a chess game. Can a "book" move follow non-"book" moves? The engine labels 4. ... Nf6 as a "book" move. Isn't "book" moves necessary an exact initial sequence of moves, but it's all about positions?

Exactly, it is a direct transposition.  To understand opening theory, you must understand the concept of transpositions.

1.Nf3 e5?? 2.Nxe5 d6 3.Nxf7?? Kxf7 4.e4 Nf6 is the exact same position as 1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nxf7?! Kxf7, so despite 1.Nf3, you are now in a Cochrane Gambit.

Other common examples of transpositions.

1.e4 d5 2.exd5 Nf6 3.c4 c6 4.d4 cxd5 is the same as 1.e4 c6 2.d4 d5 3.exd5 cxd5 4.c4 Nf6, and yes, despite 1.e4 d5, you are no longer in a Scandinavian Defense, but rather, a Caro-Kann Defense, and there are lines here that can directly transpose to the Nimzo-Indian Defense, believe it or not!

1.Nf3 Nf6 2.c4 g6 3.Nc3 Bg7 4.e4 d6 5.d4 is a King's Indian Defense, not an English Opening.

1.Nf3 d5 2.c4 e6 3.g3 Nf6 4.Bg2 Be7 5.O-O O-O and now 6.b3 is a Reti, but 6.d4 is a Catalan.

Once you transpose to a different opening, whether or not you are still "in book" is based on the new opening you transposed to, not the original move order.

Of course it's about positions, not exact sequences. When you try to find the best move, earlier moves do not matter at all.

so what is yours conclusion about this ?

The conclusion is that nobody ever said: I would know the theory here, if only this position was reached by a different move order.

or h4 d6 e4 is now a Petruccioli Attack : Pirc not a Kadas 

or f4 d5 c4 is now  a Birds Opening : Dutch, Strum Gambit which is basically the reversed Vampire 

It is all about positions.

hi

i think it is not proper using of terms.. confusing. 

book move - is a part of a way (best) to reach some game openings. 

some of game openings has Transpositions - so it means there is a different ways to reach this opening position.

IMHO conclusion is > books moves is a strict path. if you do something other > you out of book moves. 

but chess opening > may have alternative entrances with same outcome

No, that doesn't make sense. Again, nobody cares how a position was reached when trying to find the best move. Why would someone invent useless terminology? Did you ever hear someone say: this position has been reached 500 times, but there is no book move because the move order has never been played before?