A curious thing happened to Tobias Hill the other day. He was scanning the internet and noticed a book of his poems for sale. It wasn't Year of the Dog, which had a 500-copy print run and is hard to get hold of now. (He doesn't have a copy himself.) It was the more recent Zoo. "My grandmother died last year," Hill explains, "and the book being auctioned was her copy with the dedication in it. I thought, 'Oh, should I buy it back?' It did feel quite strange."
That sounds like something from one of Hill's novels: a desirable object hovering in the labyrinth of cyberspace, which proffers all sorts of possibilities for pursuit and possession. In Underground, the protagonist chases a girl through a network of Tube stations and subterranean tunnels. A collector in The Love of Stones becomes obsessed with tracking a jewel. In the newly published The Cryptographer (Faber & Faber, £10.99), the world of codes leads a tax inspector into a merry dance in her quest for information.
Secrets are the recurring theme in Hill's work, from what lurks below London's pavements to the hidden messages in a man's tattoos. Hill is not a reader of mysteries himself but he enjoys writing with "some form of resolution". It's as if he gives the reader a ball of wool on page one and, like Polonius in Hamlet, tells them to "By indirection find direction out".
"The problem with themes is that writers don't realise they are themes until someone points them out," says Hill. He sits with a cup of coffee, bare feet up on a chair in his converted railway cottage. The images Hill works with - tunnels, codes, memory - give his books an archaeological feel, I suggest. "That's an odd word," he says, nodding. "Because archaeology's what the next book is going to be about." It makes sense. There's plenty of scope for peeling back layers, following false leads, piecing together the bits of a jigsaw.
Possessing and being possessed are also features of Hill's works. In The Love of Stones, his androgynous heroine Katharine devotes her life to trying to locate a jewel called The Three Brethren. Hill has also contributed a story - "Impossible Things" - to The Phantom Museum, an anthology accompanying the current British Museum exhibition of medical curiosities belonging to the Victorian collector Henry Wellcome (published by Profile Books, £12.99).
Hill understands the passion for accumulation. His German-Jewish maternal grandmother collected semi-precious stones. "As a child l was fascinated by gemstones, in the way that small children are fascinated by dinosaurs or trains. Stones seem very physical and look like sweets. You can look at them microscopically and imagine things about them."
He started writing poetry aged 12. "At school I was never given a sense that poetry was something flowery or light. It's a complex and controlled way of using language. Rhythms and the music of it are very important. But the difficulty is that poetry makes some kind of claim of honesty. If, at a party, I say I'm a poet, people have a hard time responding, almost as if I'd said I'm a priest."
It was while travelling in South America and, later, teaching in Japan that Hill started to write poetry seriously. When his first collection won an award, he says, "It was a complete shock". Hearing about the Poetry Society residency in Regent's Park, he then applied to be the young man at the Zoo.
"It didn't involve writing poems for the animals," he says. "Fortunately". There was already somebody serenading the giraffes and penguins with verses. Apparently they loved it. Hill is laughing. Instead, he organised poetry readings in the aquarium and a workshops for aspiring poets who used the zoo as a creative springboard.
"I discovered there was a strong tradition of zoo poetry. Ted Hughes worked at London Zoo as a washer-upper and from the window could see the big cats pacing up and down." Hill wrote to Hughes for permission to use his poems in children's treasure hunts. Hughes was happy to oblige.
Before Hill had left Japan, he had attempted long narrative poems about London, which he describes as "unreadable". He smiles. "It's like trying to drink a bottle of spirits - there's far too much intensity."
So he switched to short stories and then novels. Has the poetry informed the prose? "I'm sure it has on a technical level. There was a time when people talked about poetic prose rather disparagingly, but poetic prose is to do with combing language through very cleanly and controlling it well."
Since publishing his debut novel Hill has put his poetry on hold. Each of his novels takes around two years to write. "A novel is a large thing to carry around. It feels like you're trying to do it without spilling it. The idea is still developing and you don't necessarily know where it's going. With that kind of process I find my head is simply not big enough to do more than one thing."
The ideas for the novels have sprung from different impulses. Underground was about immigration, war, and also - Hill's father having recently died - mortality. Researching The Love of Stones, Hill kept coming across pictures of an extraordinary-looking jewel, which belonged to various monarchs before vanishing during the Civil War.
"One of the things I felt when I was writing was how stones, particularly diamonds, are very cold to wear," he says. "The interest lies in who has worn them before. There's this odd kind of contrast between their coldness and the sentimental value which they have."
Hill had stumbled on two Iraqi Jewish brothers who supplied diamonds for Queen Victoria's imperial crown, and put these lapidarians into The Love of Stones as major characters. Another fictionalised historical figure appears in The Cryptographer. The shadowy John Law borrows the name and some characteristics from a Scottish rake and financier who established the first French bank in 1716. The success of Law's financial strategies proved short-lived.
The Cryptographer also acknowledges a debt to The Great Gatsby. Both his character Law and Gatsby are low-born men who get their first break at 17, become stupendously rich and lose everything. Both are enigmatic, both obsessed.
"I wanted to have fun in writing a historical science-fiction novel," explains Hill. "I also wanted to write about money - what it means to people and whether it's a good or bad thing. The real John Law had the idea that money didn't need to be gold or silver, it could just be paper. His idea was that money was trust."
The Cryptographer's John Law has made the first great electronic currency, Soft Gold, which is protected by a perfect, seemingly unbreakable code. "The more you write about money, the more you realise it's imaginary, just an idea. Trust is very important in the novel."
The Great Gatsby is also concerned with money and the failure of the American Dream in an age of "miracles, art and excess". Only 80 years on, "Money has become more sophisticated. What you could actually do with money in the past bears no comparison with what can be done now." Law, the richest man on earth, owns the parish of Erith - 4,600 acres of London - and properties on five continents. He lives like a merchant prince in the 21st century.
The Cryptographer's central character is a female tax inspector. Anna Moore is investigating Law because he has diverted money into an account with his son's name. And she falls for the inscrutable Law. A number-cruncher isn't the most obvious romantic lead, I suggest.
"Tax inspection is an interesting profession," Hill muses. "There are crack teams working on certain people. Inspectors have access in a very unusual way. There's detection involved and also a certain amount of power. And there's something about the Revenue which is slightly mysterious, sinister almost. Where is the Revenue? Who runs it? It comes back to secrets, I suppose."
Someone in the novel reflects that cryptography is the science of concealment, and that concealment can be beautiful. "It can hide the blueprint for a gun in a conversation about snow, the pattern of lights on a train, the genetic structure of a flower. But it can also undo these things. The study of concealment also concerns discovery." Sometimes you find things you didn't even know were hidden - like your grandmother's copy of your poems, floating a double-click away on the internet.
Tobias Hill was born in London in 1970, the son of a journalist. Educated at Hampstead Comprehensive, he read English at Sussex University and taught English for two years in Japan. His first poetry collection, Year of the Dog, won an Eric Gregory Award for young poets. Two further award-winning collections, Midnight in the City of Clocks and Zoo, were published by Oxford University Press. In 1998 he was the inaugural resident poet at London Zoo. His short-story collection Skin won the PEN/Macmillan Award for fiction. He has written three novels: Underground, The Love of Stones and The Cryptographer, published this week by Faber & Faber. Tobias Hill is a visiting fellow at Sussex University and lives in North London.