The author is being ironic and having a laugh at himself. He does not think reading Agatha Christie is a waste of time at all. At worst, he considers it a guilty pleasure.
Right, the attack wasn't against Agatha Christie, but on crime fiction as a genre. Called out these as an exception to the genre bring a waste of time.
My assertion is to drop that stance entirely. It turns what could be a normal compliment into a backhand one. And sets a negative tone for reading the rest.
I gather you're getting that from this single sentence:
> Quite apart from the pleasure she gives, reading her is not entirely a waste of time.
But let's look at the paragraph:
> I am a great admirer of Mrs. Christie. I enjoy her irony, and she sometimes reveals herself to be an acute psychologist. Quite apart from the pleasure she gives, reading her is not entirely a waste of time. She conveys to the reader the impression of enjoying the human comedy without bitterness or rancor, and thereby acts as an antidote to our resentment of the imperfections of the world and existence. There is also something deeply comforting about her fairy tales in which evil suddenly erupts into a pleasantly settled world only to be quickly defeated and for order to be restored. The world is not really like this, of course, and no one imagines that it is, but which of us never needs imaginative escape from reality?
That's an entirely complementary paragraph unless you read "not entirely a waste of time" to mean that it is somewhat a waste of time. But the author is clearly using "not entirely" ironically, iow, that reading Christie is not at all a waste of time.
The piece opens by declaring itself "an essay on the transcendent meaning and value of crime novels."
My assertion is to drop that stance entirely. It turns what could be a normal compliment into a backhand one. And sets a negative tone for reading the rest.