"When the Jane Austen Society has quizzes," says John Mullan, professor of English at University College London, "academics always do rather badly. One scholar implied it was rather to their credit, as if there are more important things to know about than how old Mr Collins is or how far Kellynch Hall is from Uppercross. But I take rather the opposite view, that Jane Austen pays such minute attention to these things that noticing them is a way to seeing her brilliance."

Although some people dismiss her books as just "posh people in drawing rooms", Mullan wants to make very high claims for them. Not only is Austen far superior to "contemporaries who write about many of the same things in the same sort of generic ways", he also rates her "as great a novelist as Dickens or Tolstoy or Flaubert or Henry James, and technically as great as well". And he describes "the feeling that all lovers of Jane Austen have that, while you're reading her, you become as clever as her. Unfortunately, it doesn't last very long after you put the book down. You become as sensitive to the nuances of speech and attitude and motivation as she is. It's an extraordinarily heady experience."

In his new book, What Matters in Jane Austen?: Twenty Crucial Puzzles Solved, Mullan sets out to share his enthusiasm.

The "puzzles" are of several different kinds. Few readers can have spent much time wondering whether sisters slept in the same beds as well as bedrooms in Austen's life and works - although the issue was raised in the London Review of Books in 1995, with an essay flagged up on the cover under the title "Was Jane Austen Gay?" (Help was fortunately at hand from a scholar who had consulted the records of a home furnishing store, Ring Brothers of Basingstoke, which offered reassurance that Austen's father had bought Jane and her sister Cassandra two single beds when they were young adults.)

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Often Mullan seeks to illuminate the novels by sketching in "historical details Austen's readers would have taken for granted but which are now invisible to us. Once you know a little about Regency mourning habits, for example, you see men making eyes at women while they are officially clad in black to mourn the recent death of someone they were supposed to be devoting their lives to."

On money and ageing, Mullan observes, Austen was utterly realistic. She was writing at a time when everybody had a pretty accurate sense of others' financial status, given that "inheritances were not confidential matters" and "the system of taxation made the incomes of the landed gentry widely known", while dowries might be mentioned in marriage announcements and "prize money won from capturing enemy ships was widely advertised". In such a context, it is hardly surprising that Austen is so explicit and unsentimental about how much money a couple needs to embark on matrimony.

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At 19 - "so young; known to so few" - Anne Elliot, the heroine of Persuasion, might have been sensible to turn down a particular suitor in the hope of attracting a grander and richer husband. By 22, it could be prudent to accept the same man. Meanwhile, notes Mullan, "family troubles take place in a domestic theatre with an audience of servants". Although hypochondriacs are ridiculous, effective remedies are few (for someone with a "deranged" stomach in one of her letters, Austen recommends: "You must keep him in Rhubarb & give him plenty of Port & Water"), and even minor ailments pose real risks.

Mullan even boldly addresses that most unpromising of themes, sex in Jane Austen. Although her letters cite examples of couples who have committed "sexual indiscretions" before getting married, he writes, her novels fall back on the convention that "pre-marital sex happens because a young woman falls into the hands of a rakish man, not because two people simply cannot resist each other".

After exploring a number of surprisingly revealing topics - what makes characters blush, the right way to propose, the evils of seaside resorts such as Weymouth - Mullan concludes the book with a chapter making the case for Austen as a great, highly innovative and even "experimental" novelist.

For the heroines of her predecessors, what you see is what you get: bland perfection. Fanny Burney - "probably the most accomplished woman novelist before Austen" - describes her character Cecilia as follows: "her complexion varied with every emotion of her soul, and her eyes, the heralds of her speech, now beamed with understanding and now glistened with sensibility".

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The reader, Mullan reports, is therefore treated to lots of "beaming and glistening" to demonstrate that Cecilia possesses all the right sentiments with total integrity. An Austen heroine such as Elizabeth Bennet always seems about a thousand times more real than this, since she has to "endure discovering that she has been fooled by her own feelings".

What Matters in Jane Austen? is obviously a book about a canonical author, indeed a "national treasure", and Mullan occasionally comes close to the slightly mawkish possessiveness some admirers feel about her characters, as when he notes that Gwyneth Paltrow was miscast as Emma, not only because of the colour of her eyes, but also because of a "willowy frame that seemed not to match Austen's insistence on her heroine's physical robustness". He leaves it to others to apply the insights of psychoanalytic, post-colonial or queer theory to her works, and seems unimpressed by the fact that "Eve Kosofsky Sedgwick's article, 'Jane Austen and the masturbating girl', was once the most widely read critical article about her amongst North American students".

For all this, Mullan is totally unapologetic. He believes that, although things have got much better since, something went badly wrong in much academic study of literature in the 1980s and 1990s, when wilful obscurity often masked a contempt not only for "ordinary readers" but also for the authors being studied.

"If there's an academic vice in literary criticism," he elaborates, "it is when the literary work is used as an occasion for the critic's ingenuity, so finding something really unlikely is more to be admired than finding something other people might have noticed. With that come bad habits: the condescension of posterity or triumphing over your author, as if some professor of English could be cleverer or more interesting or more insightful than Jane Austen.

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"My book is about Jane Austen's ingenuity, and if at any stage it seems to be about my ingenuity, then it has gone wrong. I'd like the reader to think it's clever of me to have noticed something, but it's always about noticing something rather than inventing something. Although I naturally hope to deepen their experience of the work, I'm not interested in getting non-academic readers of Jane Austen to say: 'Oh, I got that wrong.' Too often ingenious readings of Austen leave no room to credit the design of the novelist.

"I hope it no longer seems bizarrely fuddy-duddy, but my interest in Austen is in her as a conscious writer, not what she did despite herself or without meaning to, or what she meant to repress. I see her as someone who absolutely, minutely knew what she was doing - and that's the most interesting place for a literary critic to start."

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Postscript:

John Mullan's What Matters in Jane Austen?: Twenty Crucial Problems Solved has just been published by Bloomsbury.