Do you mean fig. 2.10? Actually he's not finding the horizon line but the vanishing points. Indeed this method is quite unpractical for traditional media but you don't have to worry, later in the book he'll teach you the Brewer method which allows you to work with distant vanishing points without having to actually draw them on the page.

Regarding your other question about the cone of vision on p. 48. The answer is also in pp. 24-25. One way to do it is like this: https://imgur.com/a/AGI4bIT

1. You draw th horizon line and then decide on any point to be the Center of Vision (CV) then draw a vertical line across this point and extend it vertically to the bottom, this line is caled the Line of Sight.

2. Place two Vanishing Points on the horizon line, one to the right and another to the left both at the same distance from the CV (They define a 90º Cone of Vision). Now you you measure 45º from each VP and draw a line until it reaches the Line of Sight, this determines your Standing Point (SP) (This represents a view from above from where the "viewer" is standing, and the horizon line now represents the Picture Plane)

3. Now you measure 30º with the center on SP and relative to the Line of Sight. Draw a line until it reaches the horizon

4. Draw a circle with its center on CV and the newly discovered point defines its radius. There you have the 60º COV!