IN PILGRIMAGE, a semi-autobiographical story about a visit to the home of the author Thomas Mann, Susan Sontag describes how, as a teenager, she used to gorge on food at tense family barbecues. "I ate and ate. How could I not?" she writes. And later: "The truth was, I dreaded conflict. And I was always hungry.”

The reference is an obvious metaphor for her insatiable appetite for books. Even as a young child she seems to have suffered from the literary version of Prader-Willi syndrome, devouring everything she could lay her hands on: "Fairy tales and comics, Compton's Encyclopedia, the Bobbsey Twins and other Stratemeyer series, books about astronomy, chemistry, China, biographies of scientists, all of Richard Halliburton's travel books and a fair number of mostly Victorian classics." This need to consume, to binge on knowledge is, it seems fair to assume, at the root of her polymathism, her love of both low and high culture and her refusal, as an adult, to limit herself to one literary genre. Sontag was, as she said herself, almost as interested in The Doors as Dostoevsky and an advocate of a polymorphous culture. Though she was best known for her essays and literary criticism, she also mastered many other forms.

Now, 13 years after her death, all her short fiction has been brought together in one volume, Stories, and what a bountiful repast it is. Indeed, in its dizzying range of structure, narrative perspective and competing ideas, and its wildly fluctuating quality, it is reminiscent of an all-you-can-eat buffet. Each dish is tempting, but soon your plate is piled so high with competing flavours, it becomes a bit daunting. And – though you may tuck in with zeal and enjoy much of what you encounter – there is a risk it will leave you bloated.

Throughout the 11 pieces, Sontag experiments freely with genre and form. The pieces in Stories run the gamut from straightforward memoir (Pilgrimage) through allegory (American Spirits) to science fiction (Dummy and Dr Jekyll) as well as a Roald Dahl-style story about pushy parents (Baby). Project for a Trip to China takes the form of disjointed notebook jottings; the Letter Scene, interwoven tableaux of various protagonists, fictional and non-fictional, penning missives of friendship, love and rejection; and Debriefing – the story of a woman's suicide – an impressionistic montage of memories and thoughts, intercut with a modernist sub-plot about the disparate lives of three women called Doris. All are well-written and interesting enough to keep the reader ploughing on, but Sontag's fascination with the avant garde sometimes produces uneven works.

A Project for a Trip to China explores the idea that a country, or the concept of a country, can exist as vividly in the imagination as it does in reality. The narrator's knowledge of China comes from her parents' recollections of it, and so her affection for the country is really an extension of her affection for them. Yet despite the poignancy of that idea, the fragmented writing keeps the reader at an emotional distance.

Baby is an entertaining satire in which two overbearing parents talk to a doctor about the "problems" they are having with their baby boomer son. He is, according to them, simultaneously a prodigy and a prospective threat (while they are simultaneously over-fussy and neglectful). The views of the doctor are never directly relayed, merely reflected in their responses, but the reader quickly intuits that the real problem lies with them. It is a clever take on parental neurosis and the generation gap, but feels like a work in progress rather than the finished article.

The best stories are the first and last: Pilgrimage and The Way We Live Now. In Pilgrimage, the 14-year-old Sontag has fallen under the spell of Mann's The Magic Mountain so her friend Merrill contacts the author who invites them both for tea. What follows is an account of that meeting which perfectly captures both her adolescent awkwardness and the realisation that great writers are not always great communicators.

The Way We Live Now is set at the height of the Aids epidemic and it captures the US at the moment when knowing someone who had the virus became the new normal. The story draws its particular power from the way it is told: through the voices of 26 friends who gather round a bedside. The patient is never named; he is neither seen nor heard, and our only insight into what he is experiencing comes from the conversations others have about him.

The story is satisfying because, as well as dealing with Aids in an inventive yet sensitive way, it examines universal themes of love and mortality. We are all defined, at least in part, by those who hold us dear; and in every serious illness there is a hierarchy of caring, with some jostling for a place at the centre of the drama, and others attempting to dissociate themselves from the heartbreak.

Its final lines are among the most striking in the collection. "I was thinking, Ursula said to Quentin, that the difference between a story and a painting or a photograph is that in a story you can write, He's still alive. But in a painting or a photo you can't show 'still.' You can just show him being alive. He's still alive, Stephen said.” I would have read the whole book just for that.