The sanatorium makes for a wonderful crucible: an enclosed space inside which the events of a novel can play out. Thomas Mann used the heightened sensibilities of a group of confined tuberculosis patients in his 1924 masterpiece, The Magic Mountain. In it, our hero, Hans Castorp, goes to visit a consumptive cousin at a sanatorium in Davos. He ends up staying for seven years, engaging in metaphysical jousting with a host of other residents – each of whom represents a different approach to the great philosophical questions of the age – and falling in love with the alluring Clawdia Chauchat. The novel ends with Castorp going off to fight (and, we presume, die) in the first world war.

AE Ellis was the pen name of the playwright Derek Lindsay, an enigmatic figure who produced one great, celebrated novel, The Rack, in 1958 and then, aside from a couple of largely forgotten plays, disappeared from view. The Rack was described by Graham Greene as rising like a “monument above the cemeteries of literature”. It is a novel in complex dialogue with Mann’s The Magic Mountain, and, to my mind at least, equally masterful. I reread The Rack with the weight of my second bout of Covid still heavy on my chest: there is now an extraordinary and dreadful resonance in a book that charts, perhaps better than anything else I’ve read, the tortuous paths of illness.

Paul Davenant is our strangely blank hero. He was orphaned as a small child (like Castorp in The Magic Mountain, like Lindsay himself) and brought up by an uncle. He was an infantry officer in the second world war and then attended Cambridge. Now 27, he is one of a group of TB patients brought to Brisset, a fictional sanatorium town in the Alps, by an international students’ organisation. Les Alpes is the name of the sanatorium in which the students will be treated, part of a pan-European charitable effort that sees them lodged alongside private patients (without enjoying the same benefits).

Davenant is the oldest and most sickly of the British students. While the others see it as an opportunity for an extended holiday, showing as they do few symptoms, and happy to be able to step into a world without obligations, Davenant is soon subjected to a host of what the sanatorium’s three doctors – Vernet, Bruneau and Florian – refer to as “tortures”. The description of the so-called cures to which, until relatively recently, TB patients were subjected is like something out of the middle ages.

The rack of the title is in part a reference to these treatments, and partly a description of the existential pangs that the patient endures. TB at this time was incurable – the best Davenant can hope for is, as one of the doctors puts it, “to obtain some compromise with the bug that will enable you to live with it on fairly amicable terms”. He is given a pneumothorax, in which the lungs are collapsed and kept artificially compressed by the regular injection of air into the chest cavity. Davenant is soon left wondering if “it would be better if he killed himself now, voluntarily, before the machine of hygiene swabbed him out of life with antiseptic and hypodermic”.

Into this world, in which the students obsess over their illnesses, discussing Shakespeare and Voltaire, Nietzsche and Molière, comes Michèle Duchesne, a 17-year-old girl with a “slender, boyish figure”. She’s referred to by one of the patients as “le petit gosse” – the little kid. Davenant falls utterly, unsettlingly in love with her. The second half of the novel is a warped, sometimes hallucinogenic love story, where we can’t help but feel that part of Davenant’s investment in Michèle is an attempt to capture some lost element of his own youth.

The Rack is not an easy read, particularly now, but it is a vital one, a novel of big ideas with a febrile, twisted sentimentality at its heart. It asks us to consider what makes life worth living, and what, in the end, we would be prepared to die for.

• Alex Preston’s latest novel, Winchelsea, is published by Faber

• The Rack by AE Ellis is published by Vintage (£10.99). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply