Ember Data breaks when fetching JSON from Node + Express REST server

I am trying to fetch JSON data from REST server built with Node.js and Express and then use it as a model in my Ember#Route.

The data I am trying to fetch:

var books = [
    { id: 98, author: 'Stanisław Lem', title: 'Solaris' },
    { id: 99, author: 'Andrzej Sapkowski', title: 'Wiedźmin' }
];

The model I use:

App.Book = DS.Model.extend({
    id: DS.attr('number'),
    author: DS.attr('string'),
    title: DS.attr('string')
});

I set up the RESTAdapter this way:

App.ApplicationAdapter = DS.RESTAdapter.extend({
    host: 'http://localhost:8080' 
});

My Router looks like this:

App.BooksRoute = Ember.Route.extend({
    model: function () {
        return this.store.find('book');
    }
});

I am aware that ember-data follows certain convention when it comes to JSON files. My server provides JSONs this way:

app.get('/books', function (request, response) {
    console.log('In GET function ');
    response.json({'books': books})
});

Then, after entering

http://localhost:8080/books

I get

{"books":[{"id":98,"author":"Stanisław Lem","title":"Solaris"},{"id":99,"author":"Andrzej Sapkowski","title":"Wiedźmin"}]}

but ember-data throws long error list that begins with:

"Error while processing route: books" "invalid 'in' operand record._attributes"    
"ember$data$lib$system$model$attributes$$getValue@http://localhost:8080/static/ember-data.js:8176:1
ember$data$lib$system$model$attributes$$attr/<@http://localhost:8080/static/ember-data.js:8202:26
computedPropertySet@http://localhost:8080/static/ember.prod.js:11882:15
computedPropertySetWithSuspend@http://localhost:8080/static/ember.prod.js:11842:9
makeCtor/Class@http://localhost:8080/static/ember.prod.js:33887:17
...

and now I don't know what is wrong and how to fix this.