I'm studying perspective using Ernest R. Norling's 1939 book Perspective Made Easy. He has a calm, firm, yet easy-going tone, and does not make learning perspective seem daunting. I especially enjoy that he states concepts simply, as if he truly wishes his readers to understand and apply them. So far, I've learned these essential points:
1. One must first locate the eye-level of a drawing. The eye-level is both the height of one's eyes and the horizon, which can be represented as a straight line across a drawing. Objects are oriented at, above, or below the eye-level.
2. Parallel lines converge at a point on the horizon called the vanishing point. Parallel lines that are parallel to the picture plane (basically one's drawing sheet held parallel to one's body and perpendicular to the ground) do not converge to a vanishing point, but remain parallel. An example is the height lines of the book in the above drawing.
3. When an object is turned, as in the book above, all parallel lines other than the height lines converge at their respective vanishing points on the same eye-level line. The vanishing points change position on this line, but the line remains unchanged.
As an exercise, I placed a book on a table and turned it to several positions. I elongated the width and length parallel lines and, if the vanishing point was within the bounds of the paper, marked it on the eye-level line. The first position shows one-point perspective: all parallel lines that are not parallel to the picture plane (my sheet) converge at one vanishing point. As I turned the book, there were two sets of parallel lines that were not parallel to my picture plane, so there were two vanishing points, most of which can be imagined as converging on the eye-level line somewhere outside of the sheet. So, what I understand is that before one begins a drawing, one must first locate the eye-level, then adjudicate whether one's object is in proper perspective by imaginatively elongating the converging parallel lines and extrapolating whether their vanishing points are indeed located on the same eye-level line.
With Bargue, I was copying from flat. Now that I'm venturing into drawing from life, I find that the hardest thing to get used to is knowing that an object is 3D, but visualizing it as 2D, as flat on my picture plane. It seems that I must see the angles not on an X-Y-Z axis, but on an X-Y axis. It's a little disorienting, but exciting.