user16626: For example, after 1. Nf3 e5?, the move 2. Nxe5 is objectively the best continuation. However, the move 2. e4 leads to a familiar opening position. In this case, do we call 2. e4 a "book move"?
Similarly, after 1. c4 d5?, is 2. d4 a "book move"?

user924: You probably won't find the position after 1.Nf3 e5? in any book. So most likely, no positions from there are book moves.
But if there's one silly book that decide to write a paragraph on 1...e5?, I'm sure it mentions only 2.Nxe5 next.
So no, just that a move would lead back into known theory doesn't make that move in that position theory.

user4418: 
For example, after 1. Nf3 e5?, the move 2. Nxe5 is objectively the best continuation. However, the move 2. e4 leads to a familiar opening position. In this case, do we call 2. e4 a "book move"?

No. Such a move is called a transposition.
According to Wikipedia:

In chess, a transposition is a sequence of moves that results in a position which may also be reached by another, more common sequence of moves. Transpositions are particularly common in the opening, where a given position may be reached by different sequences of moves. Players sometimes use transpositions deliberately, to avoid variations they dislike, lure opponents into unfamiliar or uncomfortable territory or simply to worry opponents.


16626: <p>For example, after 1. Nf3 e5?, the move 2. Nxe5 is objectively the best continuation. However, the move 2. e4 leads to a familiar opening position. In this case, do we call 2. e4 a &quot;book move&quot;?</p>
<p>Similarly, after 1. c4 d5?, is 2. d4 a &quot;book move&quot;?</p>
 924: <p>You probably won't find the position after 1.Nf3 e5? in any book. So most likely, no positions from there are book moves.</p>
<p>But if there's one silly book that decide to write a paragraph on 1...e5?, I'm sure it mentions only 2.Nxe5 next.</p>
<p>So no, just that a move would lead back into known theory doesn't make that move <em>in that position</em> theory.</p>
 4418: <blockquote>
<p>For example, after 1. Nf3 e5?, the move 2. Nxe5 is objectively the best continuation. However, the move 2. e4 leads to a familiar opening position. In this case, do we call 2. e4 a &quot;book move&quot;?</p>
</blockquote>
<p>No. Such a move is called a <em>transposition</em>.</p>
<p>According to <a href="https://en.wikipedia.org/wiki/Transposition_(chess)" rel="noreferrer">Wikipedia</a>:</p>
<blockquote>
<p>In chess, a transposition is a sequence of moves that results in a position which may also be reached by another, more common sequence of moves. Transpositions are particularly common in the opening, where a given position may be reached by different sequences of moves. Players sometimes use transpositions deliberately, to avoid variations they dislike, lure opponents into unfamiliar or uncomfortable territory or simply to worry opponents.</p>
</blockquote>
