library data modeling
Your model completely ignores the fact that books come in multiple copies.

Each copy of a book can only be in one place at one time, but each place could have multiple copies of any given book.  

Since each copy of a book will have exactly the same ISBN, the answer to your first question is: "No, ISBN is not a sufficient primary key for Books."

What you need to account for is a Title entity.  This is going to be a combination of what you're thinking about for your Books entity.  A Title is an abstract object.  It will have a title, authors, publication date, and so forth.  You might even want to consider making a distinction between Title and Edition, since a book can be republished many times with variations each time.

Whether or not you want to distinguish between editions of a title, you must at least distinguish between Title and Books.  The answer to your second question is that each physical book will be in one place (either at a Library or on loan to a Member - or in transit between branches, or lost, etc. - depending on how sophisticated you want your model to be).

On the other hand, Titles, (or Editions) will be many-to-many with locations.