Somehow the word "novel" has become synonymous with "book." I have no idea when and how this happened but it did. The fact of the matter is that they are NOT synonymous. People just think they are which is causing some unwanted problems for those of us that happen to write short stories.
All novels are books. Not all books are novels.
A book is anything that is bound and published. In the case of this ebook area, it's really just anything that's published (i.e. packaged) and sold as a single reading entity. That's why we call the things that we buy on our Kindles or Nooks eBOOKS and not eNOVELS. A collection of articles could be a book if it's packaged together. A comparative study on the flora and fauna of the Great Plains is a book. An essay, if put together and sold as an individual entity, is a book.
And, yes, a single short story is a book.
A novel is one of the major forms of literature. This means that there are certain style elements that must take place. It's usually fictional or has fictional elements. There has to be a story arc of some sort and a novel must have a certain number of words (40,000 and up is usually the standard). Once all this is said and done and you created your finished product, the novel is packaged together and sold as a book.
Hope that helped to clear things up.