What is a book? Initially, this might sound like a silly question: books are everywhere and you likely have a pretty good image in your mind of what one is. However, that idea is complicated when considering the history of the book. The object that Westerners generally call a "book" would more accurately be called a "codex," meaning a collections of pages bound together at one edge between two boards or other form of cover. Nowadays if you hear someone talking about a codex, however, they are likely referencing an older object. The codex replaced the scroll as the primary format for holding texts; logically, this means that a scroll is a book even though it diverges from the image we have today. By this logic, carved stone tablets from Mesopotamia can also be called books, and so can any other number of text-holding objects. Looking to more modern times, we refer to ebooks as "books" despite the lack of the typical physical form most would use to describe a book.
In short, there is no easy definition of what a book actually is. Many book historians have considered this question, and on this page you'll find several resources that can help you consider what makes a book, "a book."
What is book history? In short, book history is the study of books as physical objects. This includes their production (printing, binding, etc.), their movement between people (from printer to bookseller to buyer and beyond), their influence on cultures, trends in their style, and anything else that connects back to the physicality of books. Additionally, many scholars incorporate textual materials in general within the book history umbrella: things like pamphlets, comic books, sheet music, posters, etc.
This page includes sources that help explain the full scope of the field and the reasons why we study it.