user34052: I see the phrase “getting out of the book” used in a way that no longer makes sense because if I’ve seen the position before then surely anyone good has seen it thousands of times, and it’s still in the book in terms of not throwing anyone off.
Is there some authority that holds what the opening book is and updates it with classic games every year?
Or is it updated by the top positions and their win %?
A side question but related, when a pro player preps their out of book moves, they aren’t so much prepping out of book but things their opponent’s game history has never played before, correct?

user18809: Getting someone "out of the book" just means to play an opening move they're not used to or that will require them to think by themselves rather than rely on theory.
Just as today people will have analyzed different positions with different engines. In the past different people would have studied games from different sources. It's not an expression meant to be taken literally or with a "strict" sense, but rather as a way to say that a player is trying to avoid turning the game into a theoretical debate and just wants to "play some chess".

user4418: 
Is there some authority that holds what the opening book is and updates it with classic games every year?

No

Or is it updated by the top positions and their win %?

No

when a pro player preps their out of book moves, they aren’t so much prepping out of book but things their opponent’s game history has never played before, correct?

An "out of book" move is an opening novelty, a move which isn't known to theory. The current game between Caruana and van Foreest in the Tata Steel competition is a good example.
[Title "Caruana - van Foreest, Tata Steel 2023"]
[Fen ""]
[Startply "14"]

1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. d4 d5 6. Bd3 Be6 7. O-O f5 8. Qe2 Be7 9. Ne5 Nd7 10. Nxd7 Qxd7 11. f3 O-O 12. Bf4 Bf6 

Here is Chess24.com's tweet on 7...f5 with a video clip of Peter Svidler's reaction:

Jorden blitzes out 6...Be6 which is already an incredibly rare move ... I'm sure people have played it over the years ... but Be6 specifically in connection with the idea of playing f5 here is just very 2023.

For me the shocking move is 11...O-O offering the knight on e4 as a piece sacrifice but not so much for the super GMs.
Take a look at the timings from Chess24 for this part of the game:

Bd3  6s Be6 3s
O-O  1m04s f5  9s
Qe2  17m50s Be7  48s
Ne5  7m17s Nd7 33s
Nxd7  2m01s Qxd7  56s
f3 2m33s O-O  4m08s
Bf4  5m39s Bf6  27m21s
c3  45s g5 18s

So, for Caruana 6...Be6 is no big deal. He takes just over a minute for his reply. But 7...f5 is a real shock, just as it was for Svidler watching and commentating. It takes almost 18 minutes for him to work out a response. Then van Foreest takes just 48 seconds for 8...Be7 Looks like he had this position on his PC pre-game.
Van Foreest thought for just over 4 minutes over the piece sacrifice 11...O-O but Caruana needed only 5 or 6 minutes to reject it with 12. Bf4. Now van Foreest is clearly out of his prep as he takes over 27 minutes to reply.

34052: <p>I see the phrase “getting out of the book” used in a way that no longer makes sense because if I’ve seen the position before then surely anyone good has seen it thousands of times, and it’s still in the book in terms of not throwing anyone off.</p>
<p>Is there some authority that holds what the opening book is and updates it with classic games every year?</p>
<p>Or is it updated by the top positions and their win %?</p>
<p>A side question but related, when a pro player preps their out of book moves, they aren’t so much prepping out of book but things their opponent’s game history has never played before, correct?</p>
 18809: <p>Getting someone &quot;out of the book&quot; just means to play an opening move they're not used to or that will require them to think by themselves rather than rely on theory.</p>
<p>Just as today people will have analyzed different positions with different engines. In the past different people would have studied games from different sources. It's not an expression meant to be taken literally or with a &quot;strict&quot; sense, but rather as a way to say that a player is trying to avoid turning the game into a theoretical debate and just wants to &quot;play some chess&quot;.</p>
 4418: <blockquote>
<p>Is there some authority that holds what the opening book is and updates it with classic games every year?</p>
</blockquote>
<p>No</p>
<blockquote>
<p>Or is it updated by the top positions and their win %?</p>
</blockquote>
<p>No</p>
<blockquote>
<p>when a pro player preps their out of book moves, they aren’t so much prepping out of book but things their opponent’s game history has never played before, correct?</p>
</blockquote>
<p>An &quot;out of book&quot; move is an opening novelty, a move which isn't known to theory. The current game between Caruana and van Foreest in the Tata Steel competition is a good example.</p>
<pre><code>[Title &quot;Caruana - van Foreest, Tata Steel 2023&quot;]
[Fen &quot;&quot;]
[Startply &quot;14&quot;]

1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. d4 d5 6. Bd3 Be6 7. O-O f5 8. Qe2 Be7 9. Ne5 Nd7 10. Nxd7 Qxd7 11. f3 O-O 12. Bf4 Bf6 
</code></pre>
<p>Here is <a href="https://twitter.com/chess24com/status/1614977645296648192" rel="noreferrer">Chess24.com's tweet</a> on 7...f5 with a video clip of Peter Svidler's reaction:</p>
<blockquote>
<p>Jorden blitzes out 6...Be6 which is already an incredibly rare move ... I'm sure people have played it over the years ... but Be6 specifically in connection with the idea of playing f5 here is just very 2023.</p>
</blockquote>
<p>For me the shocking move is 11...O-O offering the knight on e4 as a piece sacrifice but not so much for the super GMs.</p>
<p>Take a look at the timings from Chess24 for this part of the game:</p>
<ol start="6">
<li>Bd3  6s Be6 3s</li>
<li>O-O  <strong>1m04s</strong> f5  9s</li>
<li>Qe2  <strong>17m50s</strong> Be7  48s</li>
<li>Ne5  <strong>7m17s</strong> Nd7 33s</li>
<li>Nxd7  2m01s Qxd7  56s</li>
<li>f3 2m33s O-O  <strong>4m08s</strong></li>
<li>Bf4  <strong>5m39s</strong> Bf6  <strong>27m21s</strong></li>
<li>c3  45s g5 18s</li>
</ol>
<p>So, for Caruana 6...Be6 is no big deal. He takes just over a minute for his reply. But 7...f5 is a real shock, just as it was for Svidler watching and commentating. It takes almost 18 minutes for him to work out a response. Then van Foreest takes just 48 seconds for 8...Be7 Looks like he had this position on his PC pre-game.</p>
<p>Van Foreest thought for just over 4 minutes over the piece sacrifice 11...O-O but Caruana needed only 5 or 6 minutes to reject it with 12. Bf4. Now van Foreest is clearly out of his prep as he takes over 27 minutes to reply.</p>
