Make multiple requests from array of objects and extend data with Angular

Let's say I have an array of books that I GET from an endpoint like /api/books.

[{
  name: 'Book 1',
  id: 1
}, {
  name: 'Book 2',
  id: 2
}, {
  name: 'Book 3',
  id: 3
}]

I have a BookService that retrieves the categories for an individual book. For example, this makes a request to /api/book/:id/categories.

BookService

function BookService($resource) {
  var service = $resource('/api/books', {}, {
    getAll: {
      method: 'GET'
    },
    getCategories: {
      url: '/api/book/:id/categories',
      method: 'GET',
      isArray: true
    },
  });

  return {
    getAll: getAll,
    getCategories: getCategories
  };
}

In my route, I have a resolve (using ui-router) that gets all books first and is injected into the BookCtrl:

resolve: {
  books: ['BookService', function(BookService) {
    return BookService.getAll();
  }]
}

In my example controller, I want to augment the books resolve result with each book's categories. So for example, I want books to be something like the following for my controller:

[{
  name: 'Book 1',
  id: 1,
  categories: ['adult', 'horror']
}, {
  name: 'Book 2',
  id: 2,
  categories: ['scifi']
}, {
  name: 'Book 3',
  id: 3,
  categories: ['comedy']
}]

How can I create an additional resolve (or use the already existing one) to accomplish this?

resolve: {
  books: ['BookService', function(BookService) {
    return BookService.getAll();
  }],
  bookCategories: ['BookService', 'books', function(BookService, books) {
    // this doesnt work 
    _.each(books, function(book) {
      BookService.getCategories(book.id)
        .then(function(categories) {
          book.categories = categories;
        })
    });
    // how do i return this to the controller properly?
  }],
}

function BookCtrl(books, bookCategories) {
  // maybe somsething like this?
  var booksWithCategories = _.extend(books, bookCategories);
}