Cultured, shrewd, honest and fit Commissario Guido Brunetti - the more-or-less-ideal man who is the hero of Donna Leon's hugely successful series of 26 detective novels set in Venice - begins her latest book Earthly Remains by faking a heart attack. Pushed beyond his limits in an interview with a high-end lawyer suspected of a crime, and who he believes his favourite junior colleague, Pucetti, is about to punch, Brunetti gasps for breath, collapses on his friend and ends up on the floor.

 

It is a wild gesture by the policeman who is more often in elegantly attired control. But it is characteristic, too, in that chivalry and protectiveness - both for the lawyer's young female victim and for his younger colleague - are the immediate cause. Brunetti, as this novel opens, can't take any more.

 

"I think he's got darker, he's more bothered by things," says Leon, asked whether she thinks her creation - who has admirers ranging from Ursula Le Guin to Theresa May among millions of readers in 35 countries - has changed over time. This year marks the 25th anniversary of her first Brunetti novel, and Leon, who will be 75 in September, is full of enthusiasm for life.

 

Brunetti, she points out, is still the same guy: "He reads, he has a sense of humour and irony, he's happily married, he has nice kids and a decent life, I knew when I first wrote about him that I wanted him to be someone I like." But he is not immune to the wickedness and venality that surround him, and we have seen him investigate dozens of murders in plots that, with their historical, watery settings, sometimes feel closer to fairytale than police procedural.

 

His shortlived breakdown, Leon thinks, is linked to a shift in herself. "It's because I've become darker," she says. "I come of happy people and am by nature a happy person, I wake up cheerful and go to bed cheerful but intellectually my vision is very bleak."

 

 

It turns out that the predatory lawyer and his young victim, who might have been the centre of another story, are here not the point. The moral corruption of the city is not what is bothering Leon. Instead, it is the corruption of the lagoon. The first suspicious deaths Brunetti stumbles on, once he has been packed off to recover on the nearby island of Sant'Erasmo, are those of bees. Earthly Remains - already praised in the New York Times as "one of her best" - marks Leon's coming-out as a writer of eco-detective fiction.

 

"I don't care about politics except how it will have an impact on ecology. It seems to me that more and more people in positions of power have decided they won't concern themselves with it; that global warming is inconvenient and so they won't talk about it. People with kids, I'm surprised they aren't armed. I cannot understand the passivity of people in the face of this . I get agitated."

 

Leon sounds agitated. A spry, slim figure, she talks and jokes animatedly - about opera, philanthropy, US and Italian politics, how Venice has changed in the decades since she moved there, her charmed life: "It's enjoyable because it was nothing I ever wanted - I was never driven or taught ambition as a kid. My parents just said `go get a good education, have a decent life and have fun', which was miraculously visionary for people in 1950s America."

 

But Leon, whose Spanish name is her paternal grandfather's - her other grandparents were Irish and German - has a social conscience. Her parents, who were Catholics, taught her that to vote Republican was a mortal sin and her new novel is dedicated to the liberal US supreme court justice Ruth Bader Ginsburg.

 

It turns out they are friends, having met when Leon took the judge and her husband Marty out to dinner in Venice as a favour, after learning that Marty was a fan.

 

Now she writes Ginsburg letters, "because it's the only place you can write things and nobody is going to know what you say. She's all that's between us and them," she adds. "I have such respect and love for her and I would have those feelings as an American even if I didn't know her. She's so brave and so smart."

 

Leon stopped reading news online the day after Trump's inauguration, and now gets her information from a variety of print sources including "my bible" - the regional daily newspaper Il Gazzettino. She supported Hillary Clinton and looks startled when I ask which Democrat she wanted to go up against Trump. Italian politics she regards as a decades-long stitch-up, the anti-establishment Beppe Grillo as a scapegoat.

 

But mainly, Leon insists, politics doesn't interest her. Her overwhelming concern is for the environment. The victim in her second novel was a public health inspector caught up in a conspiracy around the disposal of toxic waste, but more often she used to mention topics such as recycling almost as a joke. In Earthly Remains they are the whole point. The book demands that we recognise crimes against nature as just that.

 

"Can you think of anything worse? I really think it is our only problem, everything else is absolutely secondary and almost irrelevant," she continues. "I will not say that writers have an obligation to write about this - it's not my place to tell people what their obligations are. But it's a subject I do not resist . Trump is a global warming denier. The foxes have been put in the chicken coop."

 

Venice's vulnerability to rising sea levels is an inescapable fact. Leon moved there in the 1980s, having been blown away by its setting, architecture and history, but mostly because Venice is where her best friends - a couple who are jewellers - live.

 

Brought up in Bloomfield, New Jersey, by parents who had survived the depression but missed out on college educations, Leon was teaching in Iran while attempting to complete a PhD about Jane Austen when the revolution of 1978-79 interrupted her studies and her life. When her trunks were returned to her months later, following her hasty evacuation (part of it at gunpoint, on a bus), her papers were gone.

 

She was working as an advertising copywriter in New York when an acquaintance got in touch to ask if she wanted to visit Italy. When they arrived in Rome, it was love at first sight: "I was speechless with wonder and whenever I could I went back, it was an incredible realisation that these people had such lovely lives."

 

 

Leon joined the American expat fold, though it was not until she wrote her first novel, Death at La Fenice - about a world-famous conductor being poisoned with cyanide in his dressing-room - that her identity became a literary one. Nowadays Henry James's name crops up often in Leon's books, usually when the aristocratic Paola is ignoring her husband Brunetti because she is buried in one of his novels (the climactic section of The Wings of the Dove is set in the kind of palazzo in which Paola grew up). Brunetti, like Leon, prefers to spend his spare time with ancient Greeks and Romans. Usually, Leon explains, they are reading the same thing. It is well known that her own novels have never been translated into Italian, because she regards celebrity as irksome and doesn't want locals to read them.

 

Venice's conservation and housing issues - the acqua alta that sees much of the city regularly flooded during the winter, the scandals and delays associated with construction of the Mose tidal barrier, and the overwhelming impact of mass tourism - have long been the backdrop to Leon's novels, as they are to life in her adopted city. But in Earthly Remains the resulting dissatisfaction has reached a new pitch. In an early scene, Guido and Paola navigate the overcrowded Rialto bridge on the verge of panic.

 

 

 

 

"That happened to me," says Leon. "I was going down to Rialto one day, it must have been a Saturday because it was like this" - she tucks her elbows into her sides in a mime of being squashed - "and someone bumped into me somehow so that I caught my foot and I would have tripped only I couldn't fall because the crowd was so thick. I'm not being hysterical when I say it's unbearable."

 

Two years ago, she left. Though she still spends around a week each month in Venice, she now mostly lives in Switzerland, where she has a home in Zurich and another in the mountains. A bestseller in German before she was widely known elsewhere, she credits her Swiss publisher, the family-owned Diogenes, with having "made my career".

 

Leon is single, and feels this suits her. "I think most people profit immeasurably from marriage in every sense, but I'm too restless," she says. Her recent moves firmed up her determination to shed as much "stuff" - including money - as she can.

 

"I don't want to be didactic but I think if one has been lucky fiscally, one should give a lot of it back, because we all of us have too much," she says. Her great passion is baroque music. She discovered her love of Handel when she saw Alcina at Carnegie Hall in the 1970s and, while she is vague about the extent of her financial commitments, Leon takes evident pleasure in her role as a patron of the orchestra Il Pomo d'Oro. She had "great and glorious fun" supporting the recordings of Handel operas with her friend, the conductor and harpsichordist Alan Curtis, and is strongly of the view that opera houses need to work harder to find new, younger audiences.

 

But she won't give up writing, she says, "as long as it's fun". Recently she very much enjoyed writing a scene in which Brunetti and his dull boss Patta bond over some hand-sewn buttonholes. "I'm interested in why people do things. Crime in itself isn't interesting, it's just horrible. The convolutions of greed are more interesting intellectually than passion, because with passion the name is the answer. What happens once you open the door to temptation and to possibility, that's what fascinates me - how people worsen."

 

[Susanna Rustin is a writer and editor on the Saturday Guardian. She tweets @SusannaRustin]