Good letterforms are designed to give a lively, even texture, but
careless spacing of letters, lines and words can tear this fabric apart. 
?RobertBringhurst

The density of texture in a written or typeset page is called its color. This has nothing to do with red or green ink; it refers only to the darkness or blackness of the letterforms in mass. Once the demands of legibility and logical order are satisfi ed, evenness of color is the typographer?s normal aim. And color depends on four things: the design of the type, the spacing between the letters, the spacing between the words, and the spacing between lines. None is independent of the others. 
? Robert Bringhurst 

Just as overlong lines tire, so do overshort ones. The eye finds the long line strenuous to read because too much energy must be spent keeping the horizontal line in sight over a long distance. In the case of the too short line, the eye is compelled to change lines too often and this again wastes energy. 
? Josef Müller-Brockmann