Since 1970 or so, the librarian community has used the term “informational books” to describe everything in the nonfiction section except poetry, drama, and folktales.
Beginning in the 1980s and solidified in 2000 by a landmark paper by Nell Duke, the literacy education community has used the term “informational books” to describe a narrow subset of nonfiction books that present information about science, history, and other content areas. According to this definition, informational books do not include biography, how-to descriptions, or any kind of narrative writing. This definition is roughly equivalent to the more useful term “expository nonfiction.”
In 2010, the Common Core State Standards introduced a third definition that is much broader. It includes all narrative and expository nonfiction books plus reference books, directions, forms, maps, persuasive essays, etc.
In my opinion, if a term doesn’t have a standard, universal definition, then it’s useless. That’s why I avoid using “informational book.” I just use "nonfiction."
And yet, it seems like the confusing term is here to stay, so it’s important to know all its possible meanings.