How can I get 'pk' or 'id' in `get_context_data` from CBV?

You can get it from self.kwargs['pk'].

I'm not sure why you want to, though, since the superclass already gets the Book corresponding to that pk - that's the whole point of a DetailView.

class MyDetail(DetailView):
    model = Book
    template_name = 'book.html'

    def get_context_data(self, **kwargs):
            context = super(MyDetail, self).get_context_data(**kwargs)
            context['something'] =Book.objects.filter(pk=self.kwargs.get('pk'))
            return context

In get_context_data you already have the object in self.object (and you can do self.object.pk). Here's what happens upstream in the class hierarchy (DetailView inherits from BaseDetailView):

class BaseDetailView(SingleObjectMixin, View):
"""
A base view for displaying a single object
"""
def get(self, request, *args, **kwargs):
    self.object = self.get_object()
    context = self.get_context_data(object=self.object)
    return self.render_to_response(context)

Reading Django source code to understand stuff is incredibly easy.

And by the way, I am not sure you can always rely on the fact that kwargs has a 'pk' key.

self.kwargs['pk'] it doesn't work in Django 2.2

in DetailView

self.object is the object that this view is displaying.

So, to access the object's fields like id or pk just self.object.id or self.object.pk

So, The answer in Django 2.2 can be like:

class MyDetail(DetailView):
    model = Book
    template_name = 'book.html'

    def get_context_data(self, **kwargs):
            context = super().get_context_data(**kwargs)
            context['something'] = Book.objects.filter(pk=self.object.pk)    # <<<---
            return context

Django 2.2 Doc