* They are bound volumes of printed material. This is the traditional definition of a book.
* They contain organized information for reference. This is what differentiates them from other types of books, like novels or poetry collections.
* They are often designed for repeated use. Unlike a novel, a reference book is intended to be consulted again and again, making it similar to other types of "books" in its purpose.
Essentially, the word "book" describes the physical form of the material and its general purpose. While reference books have unique characteristics, they still fit within the broader definition of a book.