  We summarize a book under publication with his title written by the three
present authors, on the theory of Zipf's law, and more generally of power laws,
driven by the mechanism of proportional growth. The preprint is available upon
request from the authors.
  For clarity, consistence of language and conciseness, we discuss the origin
and conditions of the validity of Zipf's law using the terminology of firms'
asset values. We use firms at the entities whose size distributions are to be
explained. It should be noted, however, that most of the relations discussed in
this book, especially the intimate connection between Zipf's and Gilbrat's
laws, underlie Zipf's law in diverse scientific areas. The same models and
variations thereof can be straightforwardly applied to any of the other domains
of application.
