[sliverstorm]

It depends on the book. Take "On the Origin of Species". The core idea can be summarized in a few sentences. So why was it written? The reason you make something like that a full book is to recount experiences, anecdotes, speculation, experiments, etc to further support your core ideas. Unlike math, many fields are based on ideas that cannot stand on their own, and once upon a time there were no journals to publish in.