What determines what an author includes in an in formational book?

There are a lot of ways to answer this question, ranging from what it takes to do justice to the subject to what it takes to fill a niche in the market.

From the point of view of the author, it might be a matter of the author's own expertise or area of interest. The problem is probably going to be in narrowing rather than broadening a subject. Research tends to bring up new questions and open doors to further exploration. To an interested person, every new avenue is exciting and leads to still more avenues.

Depending on the type of book and the type of audience it's intended for, factors in content selection might be how deep or detailed it is meant to go, whether it is meant to inform, entertain, teach, or persuade, how slanted it is going to be toward a certain point of view, and how big (how long) it is intended to be. The author could be trying to sum up major trends in the history of the world in 100 pages, or devoting 500 pages to the evolutionary history, life cycle, and behavior of a certain kind of snail. Those goals would produce extremely different results.

Decisions about scope of content are also going to involve the publisher (assuming that we're talking about a book that is getting published). In fact, a publisher might start the process by wanting to add coverage of a certain subject to its list of titles and seeking out an author for a book it wants to publish. This might especially be the case for textbooks.

And the publishing question gets into marketing: what other books on the subject are out there? What is the competition like? Is this book going to try to improve on something that's already there or supply something new? What will it take to sell the book? For instance, if it's a book about a celebrity, are there pictures that have never been published before, a new source to interview, letters that have just been uncovered?

An ethical, objective, fair-minded author might try to give a balanced presentation of a subject, one that takes all legitimate points of view into account. A sensationalist author might look for what is most likely to grab public attention and not worry too much about fine points of proof and opposing views.

Ultimately there will be some sort of purpose or goal associated with a writing project, whatever it is that is driving the author--a passion for the truth, personal ambition, professional achievement, a desire for fame and fortune and talk show appearances, hack work for hire, whatever--and that goal will be the yardstick that determines everything about the book from the first outline to the cover design.