Normal position of Electrical axis only in the III lead?
Let's consider the definition of the electrical axis.
The text does not specify which type of electrical axis and of what.
For instance, Guyton discusses Mean Electrical Axis of the Ventricular QRS only.
However, Guyton says that the mean electrical axis of the normal ventricle is 59 degrees. 
In many pathological conditions of the heart, this direction changes markedly, sometimes even to opposite poles of the heart.

So the original author must have considered the Mean Electrical Axis of Ventricular QRS.

I would change the paragraph to


  Mean Electrical axes of the Ventricular QRS
  
  
  Excitatory processes have definite direction and magnitude and could
  therefore be represented as vectors. At any moment there are two
  points in the heart with the biggest potential difference, which is
  called electrical axis, characterized by the angel α. 
  Under normal
  conditions -> 30°&lt; angel α &lt; 90° 
  The Mean Electrical axis of normal ventricles is 59 degrees, which gives information
  about the position of the heart, and it represents the biggest amplitude in the lead III.