ember.js ( 2.5) ember routing issue with belongsTo relationship

In my ember app, I am displaying a list of books

    BOOKS
    Book                                Author             
    Gorgeous Metal Fish Cookbook        Jefferey Gibson 

I am trying to display author's information ( in a modal window) upon click on the author's name in my books template, I have an action 'showAuthor' , defined in the the books route, but I get undefined data...

<span ></span>


    **TEMPLATES**
    <!-- app/templates/books.hbs -->
     <h1>Books</h1>
     <table class="table table-bordered table-striped">
       <thead>
         . . .
       </thead>
       <tbody>
         
           <tr>
             <td>
               . . .
            </td>
             <td>
               <span ></span>
             </td>
           </tr>
         
       </tbody>
     </table>


    ============
    **MODELS**
    // app/models/book.js
    export default Model.extend({
        . . .
          author:       belongsTo('author', {inverse: 'books', async: true}),
        . . .
    });

    // app/models/author.js
    export default Model.extend({
        . . .
      books: hasMany('book', {inverse: 'author', async: true}),
        . . .
    });

    =========
    **ROUTES**
    app/routes/books.js    
    import Ember from 'ember';
    export default Ember.Route.extend({
      model() {
        return this.store.findAll('book');
      },
      actions: { 
        .. .
       showBookAuthor(book){
          alert('SHOW BOOK AUTHOR : ' + book.author.name);
        }, 
       . . .
      }
    });


    app/routes/authors.js    
    import Ember from 'ember';
    export default Ember.Route.extend({
      model() {
        return this.store.findAll('author');
      },
      actions: { 
        .. .
      }
    });