Worth quoting the preface in full, as it elegantly gives you a feeling for the rest of this often quoted book:

Illness is the night-side of life, a more onerous citizenship. Everyone who is born holds dual citizenship, in the kingdom of the well and the kingdom of the sick. Although we all prefer to use only the good passport, sooner or later each of us is obliged, at least for a spell, to identify ourselves as citizens of that other place.

I want to describe, not what it is really like to emigrate to the kingdom of the ill and live there, but the punitive or sentimental fantasies concocted about the situation: not real geography, but stereotypes of national character. My subject is not physical illness itself but the uses of illness itself as a figure or metaphor. My point is that illness is not a metaphor, and that the most truthful way of regarding illness – and the healthiest way of being ill – is one most purified of, most resistant to, metaphoric thinking. Yet it is hardly possible to take up one’s residence in the kingdom of the ill unprejudiced by the lurid metaphors with which it has been landscaped. It is toward an elucidation of those metaphors, and a liberation from them, that I dedicate this inquiry.

Musing on this, Sontag skillfully lays bare a lot of the ways in which we deceive ourselves, and how society creates other fictions around two principal illnesses: TB and cancer. She traces back the and contrasts the Romantic obsession with TB especially. There’s a clarity to this essay: she debunks some of the ways we think about illness, especially cancer; how e canalzily attribute moral or emotional categories to the illness, which in reality might have nothing to do with how it has come about e.g. someone might develop cancer because they’ve been ‘repressed’ emotionally.

However, interesting critique of the essay to be found here, which poses the question that perhaps Sontag should have been kinder in looking at metaphor a way of making meaning in life; however erroneous, it is a part of how we sometimes use narratives to make sense of things. I’ve seen enough serious illness, including cancer, to be convinced that the stories that we tell ourselves can be a source of both comfort and despair, that narrative identity is important. The opening lines of the essay themselves are a masterclass in metaphor, which was probably her point.