I nearly always read three books in parallel and if one is finished it is replaced by another of the same kind:
One is invariably easy to read fiction (crime, for instance. At this moment, a Mankell book.
The next is about parasciences, sometimes pro, mostly contra. At this time, it is a book about parapsychology.
The third is either a science book or political nonfiction. At this time, the third is a book about Al Quaeda.

A fourth book is only added, if it is a book that I do not read each page of, like for instance a new songbook.

Wolfgang