<!-- Nav tabs -->
  <ul class="nav nav-tabs">
    <?php
      $panel_id = 1;
      $args = array(
        'post_type' => 'book',
        'posts_per_page' => 8,
        'orderby' => 'date',
        'order' => 'ASC',
      );
      $book = new WP_Query( $args );
      while( $book->have_posts() ) : $book->the_post();
    ?>
    <li<?php if ( $book->current_post == 0 ): ?> class="active"<?php endif; ?>>
      <a href="#<?php echo $panel_id; ?>" data-toggle="tab"><?php the_title(); ?></a>
    </li>

    <?php $panel_id++; endwhile;  wp_reset_postdata(); ?>
  </ul>
  <!-- Tab panes -->
  <div class="tab-content">
    <?php
      $panel_id = 1;
      $args = array(
        'post_type' => 'book',
        'posts_per_page' => 8,
        'orderby' => 'date',
        'order' => 'ASC',
      );
      $book_2 = new WP_Query( $args );
      while( $book_2->have_posts() ) : $book_2->the_post();
    ?>
    <div class="tab-pane fade<?php if ( $book_2->current_post == 0 ): ?> in active<?php endif; ?>" id="<?php echo $panel_id; ?>">
      <div class="row">
        <div class="col-xs-4 col-sm-2">
          <?php the_post_thumbnail('thumbnail', array('class' => 'img-responsive img-thumbnail')); ?>
        </div>
        <div class="col-xs-8 col-sm-10">
          <?php the_content(); ?>
        </div>
      </div>
    </div>