Before we start to design our application, let's have a good look at the different entities that need to be handled by it. The entities we recognize are a book, an author, and a user. A book may have many attributes, but here we limit ourselves to a title, an ISBN (International Standard Book Number), and a publisher. An author has just a name, but of course, if we would like to extend that with extra attributes, like the date of birth or nationality, we can always add that later. Finally, a user is an entity with a single attribute as well, the user ID.
The next important part is to have a clear understanding of the relations between these entities. A book may be written by one or more authors, so we need to define a relation between a book entity and an author entity. Also, any number of users may own a copy of a book. This is another relation we have to define, this time, between a book entity and a user entity. The following diagram may help to see those entities...