I used a counter to solve this. Found a solution for the gallery field that worked the same way.
Here you go!
<?php
$posts = get_field('books');
$max = 3;
$i = 0;
if( $posts ): ?>
<?php foreach( $posts as $post): $i++; ?>
<?php if( $i > $max){ break; } ?>
<?php setup_postdata($post); ?>
<div class="col-xs-6 col-sm-3">
<a href="<?php the_permalink(); ?>">
<?php if (has_post_thumbnail( $post->ID ) ): ?>
<?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'book-thumbnail' ); ?>
<img class="img-responsive" src="<?php echo $image[0]; ?>" alt="<?php the_title(); ?>">
<?php endif; ?>
</a>
</div>
<?php endforeach; ?>
<?php endif; ?>
