[Django]-How do I iterate ManyToMany field in Django template tag?

It’s because if you call b.category it returns only the relation object. To get its values (category objects) you have to add .all. Like this:

{% for b in books %}
<div>
    {{ b.title }}
    {% for cat in b.category.all %}
        {{cat}}
    {% endfor %}
</div>
{% endfor %}

By the way, I’ve also changed c.title to b.title, because I assume you want this book title, not something from global.