Pios88

$authors = Author::find()->with('book')->all();
foreach($authors as $author){
         echo $author->name;
         echo '<br>';
         if($author->book){
              echo 'у автора ' . count($author->name) . 'книг';
              foreach($author->book as $book){
                  echo $book->title;
                  echo '<br>';
               }
          }else{
                echo 'этот дядя не писал книг';
                echo '<br>';
         }
}

class AuthorController extends Controller {
    public function actionIndex() {
        $authors = Author::find()->select(['author.id', 'author.name', 'books' => 'count(*)'])
            ->leftJoin('book', 'author.id = book.author_id')
            ->groupBy('author.id')->asArray()->all();
        return $this->render('index', ['authors' => $authors]);   
    }
}

foreach($authors as $author){
  echo '<h2>'.$author['id'].') '.$author['name'].'</h2>'; // 1) Иван И.И.
  echo '<p>Книг: '.$author['books'].'</p>'; // Книг: 17
}

<?php
class Author extends ActiveRecord
{
    public static function tableName()
    {
       return 'author';
    }

    pubic static function primaryKey()
    {
        return ['id'];
    }
}

class Book extends ActiveRecord
{
    public static function tableName()
    {
       return 'book';
    }

    public static function primaryKey()
    {
        return ['id'];
    }

    public function getAuthor()
    {
        return $this->hasOne(Author::className(), ['id' => 'author_id']);
    }
}

// Использование:
$books = Book::find()->all();
foreach ($books as $book) {
   echo $book->author->name; // сработает метод getAuthor() из Book
}