user16626: In the opening stage, are there any moves that are perfectly sound but are not book moves? Here a book move, is defined in Wikipedia as follows: 

Opening moves that are considered standard (often catalogued in a reference work such as the Encyclopaedia of Chess Openings) are referred to as "book moves", or simply "book". Reference works often present move sequences in simple algebraic notation, opening trees, or theory tables.

By a sound move, I mean a move that is completely reasonable in a certain opening position and does not create any weaknesses. (There may be a better definition, but you probably know what I mean.)

user13230: Obviously, you are not the first chess player to think about alternatives to the 'book moves'.
I think it is a solid assumption that all possible moves that deviate from the book move have been considered by someone already, and probably by someone with some playing strength.
So either those moves are good enough (equivalent) to 'book moves' and therefore they would have made it in the books by now, or they are (considered) at least slightly inferior.
If you consider playing an inferior move, you will have to balance the expected 'loss' from the inferiority against the potential 'gain' from forcing your opponent into a non-book variant (which you have analyzed in depth before, and he probably didn't). Depending on his strength in 'book openings' compared to his strength in 'think-yourself', this might be a good deal for you or not.
Note that this is not an uncommon idea - nearly all chess players I know have taken this approach from time to time, and more or less successful. Try it, but don't expect your opponent to be shocked out of his mind.

16626: <p>In the opening stage, are there any moves that are perfectly sound but are not book moves? Here a book move, is defined in Wikipedia as follows: </p>

<blockquote>
  <p>Opening moves that are considered standard (often catalogued in a reference work such as the Encyclopaedia of Chess Openings) are referred to as "book moves", or simply "book". Reference works often present move sequences in simple algebraic notation, opening trees, or theory tables.</p>
</blockquote>

<p>By a sound move, I mean a move that is completely reasonable in a certain opening position and does not create any weaknesses. (There may be a better definition, but you probably know what I mean.)</p>
 13230: <p>Obviously, you are not the first chess player to think about alternatives to the 'book moves'.</p>

<p>I think it is a solid assumption that all possible moves that deviate from the book move have been considered by someone already, and probably by someone with some playing strength.<br>
So either those moves are good enough (equivalent) to 'book moves' and therefore they would have made it in the books by now, or they are (considered) at least slightly inferior.</p>

<p>If you consider playing an inferior move, you will have to balance the expected 'loss' from the inferiority against the potential 'gain' from forcing your opponent into a non-book variant (which <em>you</em> have analyzed in depth before, and he probably didn't). Depending on his strength in 'book openings' compared to his strength in 'think-yourself', this might be a good deal for you or not.</p>

<p>Note that this is not an uncommon idea - nearly all chess players I know have taken this approach from time to time, and more or less successful. Try it, but don't expect your opponent to be shocked out of his mind.</p>
