In 1843, two years before her death at the age of seventy-two, Cassandra Austen told her brother Charles that she had been ‘looking over & destroying some of my Papers’, but was keeping ‘a few letters & a few Manuscripts of our dear Jane, which I have set apart for those parties to whom I think they will be mostly valuable’. What survived the bonfires makes pretty thin reading, and Jane Austen devotees have often speculated about the lost bits of the record. Was the great satirist really such a bland homebody? How could she have concocted some of the world’s favourite love stories and never been passionately in love herself? Was there not a Darcy, a Knightley, or (more likely) an Edmund Bertram or Captain Wentworth lurking in her own past?

Gill Hornby has taken these questions and turned them, delightfully and ingeniously, into a novel written from the point of view of the lesser-known Miss Austen, using the relationship between the sisters as a key to the novelist’s sensibility. The story starts with, and circles around, Cassandra’s engagement to Tom Fowle in 1795, when everything looked on course for a traditional happy ending. But Tom’s sudden death in the West Indies plunged Cassandra into a sort of singleton widowhood, following which her sister Jane, equally loyal and intransigent, resolved to remain her companion. They stayed together ever after, an arrangement that proved more successful than most marriages; Cassandra had a small legacy that made her almost independent and Jane was able to pursue in peace her strange desire to write novels.

The story so far is historically accurate. Hornby then transports us to 1840: Jane has been dead for twenty-three years and Tom’s former family home in Kintbury is about to be vacated, sending the now elderly Cassandra into a panic when she thinks what old letters may be hidden there. She rolls up at the house, somewhat to the bemusement of her relations, secretly hoping to find and destroy anything that could compromise Jane’s reputation. After a bit of creeping around, riffling through bureaus and rousing the suspicions of a grumpy maid named Dinah, Cassandra manages to find a hoard of letters to her old friend Eliza Fowle, and the past comes flooding back to her. Or does it? What actually happened at Sidmouth in 1801? Whose version of events is right? And what lay behind Jane’s decision to break off her own engagement in 1802?

While Cassandra trespasses on the past, life in Kintbury proves full of half-concealed tensions and intrigues. Tom Fowle’s spinster niece Isabella, who nursed her old parents, now faces becoming dependent herself, while her widowed sister Mary-Jane has become a bit of a joke in the village, a deep-voiced pipe smoker with a taste for dervish dancing. Then Mary Austen turns up to give us a blast of amusing unpleasantness, droning on about the superiority of her husband’s poems to anything his sister Jane managed to write and patronising everyone in sight. Jane’s droll prediction that Mary would never spot herself as the model for Mary Bennet (and possibly Mary Musgrove and Fanny Dashwood too) seems to have come true.

As you’d imagine, there are plenty of references to Jane’s novels and letters, but faced with some tricky choices about how much to signal to Austen superfans, Hornby manages to stay basically credible (unlike Andrew Davies’s recent Sanditon fantasia) and does amazingly well in the riskiest area of all, the invention of letters ostensibly written by Jane. If anything, these are possibly a bit too funny: ‘Mr and Mrs G.A. are determined on a glorious retirement,’ Jane writes from Bath, ‘while we young ladies are promised to be met with splendid suitors in an endless array. We shall see. But should that all happen, I give you fair warning: I shall ignore all evidence of character or appearance or the good of our families and instead plump at once for the richest.’ Cassandra remains a rather stiff and priggish character, but Hornby’s Jane really seems to live and breathe, has mood swings and lapses of patience, articulates clever ideas (she has a theory that the mothers of spurned suitors are likely to keep in touch in case their first daughter-in-law dies through childbearing), laughs and cries. Even when the plot seems a bit rickety, this creative gloss on ‘dear Jane’ powers through.

The television rights to this novel were sold at birth. No surprise: the dialogue is ready to roll, there’s a treasure hunt, carriages and libraries, dogs and swishing canes, and even a girls’ hair-brushing scene. People are going to love it, but I wonder if any screen adaptation will be able to convey the hidden treasure within this thoughtful story: the powerful, barely expressed knowledge of each other’s characters that both the Misses Austen took to their graves.