user12396: This is a sample position from a book (no further details are given to avoid copyright issues).

I can read the first line and follow it in my mind, departing from the figure. But the continuation in the 2nd and 3rd line is too much. I cannot look at the figure and remember what pieces are supposed to be in different squares, much less do it in a proficient way (i.e. I might place a non-existing bishop in a square in my mind, but not realizing which other squares he is threatening).
However the book is not for advanced players. I wonder how are readers supposed to follow these comments and variations, if you are supposed to use a physical board with pieces, or if you are supposed to visualize all that in your mind and the problem is that I need more practice on that.

user9025: Visualization usually improves as you get stronger, and there are exercises that can be done to improve it. 
When following lines in a book, most players use a board,  and sometimes two. The second one is used to explore those side variations,  without having to constantly reset the board and play from the first move. This can be done with either physical boards, or software, which makes it easier to follow a variation,  then go back to then main line with just a mouse click. 

12396: <p>This is a sample position from a book (no further details are given to avoid copyright issues).</p>

<p><a href="https://i.stack.imgur.com/9Vybj.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9Vybj.jpg" alt="enter image description here"></a></p>

<p>I can read the first line and follow it in my mind, departing from the figure. But the continuation in the 2nd and 3rd line is too much. I cannot look at the figure and remember what pieces are supposed to be in different squares, much less do it in a proficient way (i.e. I might place a non-existing bishop in a square in my mind, but not realizing which other squares he is threatening).</p>

<p>However the book is not for advanced players. I wonder how are readers supposed to follow these comments and variations, if you are supposed to use a physical board with pieces, or if you are supposed to visualize all that in your mind and the problem is that I need more practice on that.</p>
 9025: <p>Visualization usually improves as you get stronger, and there are exercises that can be done to improve it. </p>

<p>When following lines in a book, most players use a board,  and sometimes two. The second one is used to explore those side variations,  without having to constantly reset the board and play from the first move. This can be done with either physical boards, or software, which makes it easier to follow a variation,  then go back to then main line with just a mouse click. </p>
