What determines the end of book moves and how can I see a "book move continuation" to a game

Hi,

Sorry for the confusing title.

Most games start with book moves, but there is a point when my moves stop being considered as "book moves", even when my following move is the one the engine considers the best. This happened in my last game:

My move c3 was the last book move and d4 was considered the "best move". Why is this? Is it because my opponent had an "inaccuracy" with the move h6, after my last book move? If his move was in the chess.com database, would both our moves still be considered as "book"?

I'd like to understand the concept of "book moves" a little better.

Additionally, how can I find "book move" outcomes to this game?

Thanks in advance. 
 

A book move is a move that has been played before and that has been recorded in a book.
As long as you follow some previously played game you play book moves and as soon as you deviate from previously played games you are out of book.