When to publish a journal article versus a scholarly book? In some fields, original research is reported almost exclusively in journal articles, but in others scholarly books are more common.

In the latter fields, how does one decide whether to publish a journal article or wait and introduce the research in a longer book?
My observation is that in those disciplines one decides before hand to do research to write a book. So it is not a matter you have results and wait with publishing it, but as long as the book is not done, the research is not considered to be done.