A thorough and beautiful guide to typography and typesetting, worthy of any designer’s desk.
A good definition of readability:
A number of typographic conventions have evolved to make text easier to read. In the context of book design, these are mainly concerned with continuous text, that is, long passages of text set in lines to form a block. Anything that confuses the eye or makes it stumble forms a barrier between the reader and the words.Mitchell & Wightman, Book Typography, page 18
Emphasis mine; I think this is equally true of shorter passages as well. There, the eye needs clear guidance as to how to move among texts; the layout of the page and the hierarchy in the typesetting need to guide the eye from place to place. A reading may be discontinuous, but it should not be bewildering.