Author Audrey Niffenegger introduces her new book

Time is a commodity much on Audrey Niffenegger's mind. Today, she is in London to talk about her new book, a haunting graphic novel called The Night Bookmobile, and as we sit down in a café inside the neo-classical splendour of Somerset House, I learn that she is squeezing this conversation into a fearsome publicity schedule of interviews, readings and an imminent BBC radio appearance. No wonder, then, that when I ask about the impact on her life of her all-conquering first novel, 2002's The Time Traveller's Wife, she talks about how that success has changed the way she disposes of her time:

"One major difference is that now when I make something, I spend a great deal of time promoting it," she says. "Before, I would just move on to the next thing.

"But when I'm not on the road, what success has meant is that I have control over my own time, which is great. All I ever wanted was the time to do what I wanted."

When The Time Traveller's Wife was published, Niffenegger was a 40-year-old, relatively obscure visual artist teaching at the Chicago Centre for Book and Paper Arts, and toiling over handmade craft books that she sold to private collectors. The novel was her first text-only work; picked up by Oprah's Book Club in the US and the Richard and Judy Book Club in the UK, the novel - which tells a love story between an artist, Clare, and a librarian, Henry, who is cursed to travel chaotically back and forth in time due to a rare genetic disorder - sold more than 2.5 million copies, and is inarguably one of the literary landmarks of the past decade. Naturally, this sudden, vast success came as something of a surprise: "it would be hubris to expect something like that," says Niffenegger. "As a visual artist, the big dream was just to have a normal, middle-class lifestyle, while having the time to be creative. I got used to making do, and budgeting."

A film version of Time Traveller's followed, along with a second novel in 2009, Her Fearful Symmetry.

While casual observers might view The Night Bookmobile, then, as a diversion, it really marks a return to the medium Niffenegger knows best: the graphic novel. This book tells the story of Alex, a young woman from Chicago who one night encounters a ghostly mobile library that contains every book - and only those books - that she has read. A devoted reader, Alex becomes obsessed with both finding this otherworldly, personalised bookmobile again, and with reading more in order to further stock its shelves. Soon enough, her obsession has finished off her already faltering relationship and consumed her life.

In this way, Niffenegger presents a mesmeric hymn to the power of reading. As we settle down over coffee - Niffenegger has an Americano, no milk - she explains that the story has a double genesis: in a childhood dream, and the HG Wells story Door in the Wall.

"The dream is one I've had my whole life," she explains. "I'm in my grandmother's big, rambling house, and there's this small door I hadn't known about before, and behind it is this enormous, grand library. And I know, in the way that you know things in dreams, that this room is the afterlife. I always wake and think: wouldn't it be great if heaven really was like that.

"As for Door in the Wall, it's always struck me as a perfect story. It's about a boy who finds a secret garden behind a door and becomes obsessed with getting back to that garden but never can. The end is very ambiguous and shocking; when I couldn't finish The Night Bookmobile, I thought back to this story and thought: that could work."

The story first appeared in the Guardian newspaper in the UK, before finding its way between hard covers. To create the pictures - which radiate a hand-drawn warmth - Niffenegger chose friends to play each character in her story, photographed them, and then drew from these photographs using artist's rapidograph pens on Bristol board.

"It's funny, there are bloggers saying: 'Why is Audrey Niffenegger trying to draw? This book has only been published because she is famous.' And here I am, a visual artist!"

Indeed, though the kind of fame now attached to her came via literature, Niffenegger was making a success out of her work in graphic novels long before. She spent 14 years hand-making 10 copies of one work, The Three Incestuous Sisters, and sold all 10 for $10,000 (Dh37,000) each. A printed version of that story was published in 2005: "That's been another great side effect of success," says Niffenegger, "bringing these obscure works to a wider audience. My politics demand a wide distribution, but the nature of the work is that it takes a very long time, and you have to charge accordingly."

For years, then, Niffenegger was the kind of artist - struggling, credible, committed to her creative vision - who is often sceptical about the mainstream. Sure enough, this multimillion-selling author has confessed: "I'm enough of a snob to hesitate before I pick up a copy of a book that's sold millions". Would it be fair to say, then, that she stands in a somewhat uneasy relationship to her own mainstream success?

"It bothers me if people think my books must not be any good because they've sold a lot of copies. Like many other people, I picked up The Da Vinci Code in an airport and read the first page and thought: 'Oh, this is astonishingly bad.'

"I hope people don't lump me into a category in which I don't belong, just because The Time Traveller's Wife sold."

The seriousness of Niffenegger's literary project is evident indirectly in The Night Bookmobile. It's hard to imagine a more overt, and more powerful, testimony to the importance and the magic of reading. Niffenegger says more stories that explore this world of readers, books and ghostly librarians will follow in the years ahead. But the message delivered by the story - which is, Niffenegger says, emphatically meant for adults - is far from straightforward; a deep ambiguity runs through Alex's obsession with books, and she sacrifices much to satisfy it (to say more would be to spoil the story). Readers will want to know: is there something of Audrey in Alex?

"Well, reading has been very potent in my own life, of course," she says. "I'm trying to express something that I feel about books, but Alex is a character, and we are different. Unlike Alex, for example, I love Thomas Pynchon," she smiles: Alex starts to read Pynchon's Gravity's Rainbow, but can't finish it.

Ultimately, The Night Bookmobile delivers - in a few, short pages - a moving study of the inevitable tension between the life of the mind and the world of tangible, external things. Does Niffenegger ponder that tension in her own life?

"Oh, absolutely. I knew really young, for example, that I would never have kids. I mean, what the hell would you do with them? I thought: 'I have work to do, you know? I don't have time for small children'.

"What has been interesting to me are the different reactions to the way Alex resolves her obsession. Some people have been very judgmental about it, and said that she's pathetic, she's a loser. I'm not advocating that people act like Alex because in real life there is no night bookmobile. But my feeling is that she has found something she really loves; it triumphs over everything else, and she is happy with that. Yes, she has to give something up: but that's what real life is like."