"I definitely feel liberated," she says, over the phone from New York. "It's wonderful to be able to invent after years of holding myself strictly to the truth. And it feels like a homecoming creatively as that's where I got my start."
It's also a chance to shift the focus away from herself, after her emotional and spiritual struggles were made so very public when her 2006 release Eat, Pray, Love shot to the top of The New York Times bestsellers list. The memoir of her post-divorce travels remained on the list for more than 200 weeks and sold 8.25 million copies. However, The Signature of All Things garnered mixed reviews, with some critics labelling it as "chick lit".
"I don't even give that label enough weight to want to fight it," she says. "A lot of people have - kindly - said The Signature of All Things is the book that will bring you a new level of readers. And I just get so offended on behalf of my current readers when I hear that because it's so dismissive. In the US, women constitute 80 per cent of the reading public. And I know it's a coded message; 'a better class of readers' is code for … um, men."
She thinks that she's grown creatively since her first book. "I hope so; it's been 10 years," she says, laughing. "I feel like it's the best book I've ever written, but I don't feel like it's necessarily the most important. Eat, Pray, Love remains a book that has really changed a lot of people's lives, and I've met those people and I've heard their stories, and it had an impact on people in a very direct and emotional way."
This time, the story centres on botanist Alma Whittaker, born in Philadelphia in 1800, and takes the reader on an ambitious, fast-paced journey through England, Peru, Holland and Tahiti in the 18th and 19th centuries.
Gilbert spent three years researching the period. "For a year I didn't read anything that was written after 1880. Novels, first-person accounts … and letters are incredibly valuable, because that's when you can hear people's voices. It's like eavesdropping on a conversation, especially in those times when people were letter writers," she says.
Even though the object of Alma's affections, Ambrose Pike, doesn't turn out to be the prince charming she hopes for, hers is a love story of a different kind, Gilbert says. "I wanted to write about a woman's love of her work. Because that's not a story that is often told, but it is a place where women continue to find comfort when other parts of their personal lives have not delivered."
Gilbert also wanted to take apart the classic 19th-century women's novels. "I love them, but they do have only two endings, right? You either have the good marriage to the landed gentry, or you're ruined," she says.
"I think the reality, both then and now, is that a few of us get magical princely marriages, and a few of us are ruined, but the huge majority of us women are somewhere in the very wide middle of those two. I wanted to create a woman who, at the end of her life, would look back and know that she'd had a very interesting, dignified journey."
Autobiographical references appear throughout the book, albeit in coded ways. Alma bears no children, but she sees her botanical learnings as her contribution to the world. In both Eat, Pray, Love and her subsequent memoir, Committed, Gilbert talks about the social preconceptions she has to face as a woman who has no desire to become a mother.
"My DNA is all over Alma," she says. "It actually doesn't even come out, the question of having children, which is also a reflection of how comfortable I've become with being a childless person. I mean she thinks about sex all the time, but…"
Sexual frustration provides an undercurrent of tension in the book. As a young girl, Alma educates herself thoroughly on the topic, stumbling upon erotic reading material in her father's library, but her experiences are largely limited to guilty self-exploration in a dark closet. "I hated to do it to her but it just felt like it was true," says Gilbert.
"I knew from the beginning that I wanted her to be a really carnal person, because that's something George Eliot and Jane Austen couldn't write about. It was something they could allude to, very obliquely. And you just know people were masturbating, you know they were.
"I have a friend who is a romance novelist who is contractually obliged to write 2.5 sex scenes per book, so I took her out for lunch and said, 'I'm uncertain how to proceed here', and she said, 'It's really simple. You just have to ask yourself what the character would actually do, and have them do it.' Once I had that in mind, it was very clear to me that Alma the explorer was not going to leave herself unexplored."
Alma and the ethereal Ambrose sit at opposite ends of the scale between the earthy and the divine. "They are metaphors for what was happening in the 19th century, and the division that was being created between divinity and science - a division that, not unlike their marriage, couldn't work."
So where does Gilbert - who in her memoirs jokes about trying to describe her spiritual experiences in India to sceptical New York friends - sit on that scale today?
"I'm placed right in the middle of them, next to Alfred Russell Wallace," she says, referring to the British explorer, who she brings in at the end of the book. "He managed to come up with a theory that both scientists and the religious hate equally. But he was such an unfettered, unusual thinker, I don't think we've seen his like since."
Throughout the novel, botany provides a rich source of symbolism. "The Latin name for mosses is cryptogamia, which means hidden marriage. Because for the longest time botanists didn't know how mosses had sex, and thought that they didn't have sex. Of course that's a parallel with Alma, whose sexuality is so hidden, but also a parallel with all of women's lives in the 19th century. The miniaturised world of moss also felt like a good representation of the miniaturised world of women's work, and the fact that most women through history had to find very tiny and quiet ways to express their creativity; you know, needlework, painting teacups."
It wasn't so bad getting the "Hollywood treatment" when she was portrayed by Julia Roberts in the film adaptation of Eat, Pray, Love, she says. "There's something very nice about watching 'you' making out with Javier Bardem. It does feel like you've 'made it'.
"But Eat, Pray, Love will be the godmother of all the work that I do for the rest of my life, probably. The Signature of All Things is the reflection of a feeling I had, that this great opportunity must not be wasted. I have such reverence for the writers who came before me, and hardly any of them got the break that I got. And if you're fortunate enough to be given that kind of independence, then don't respond with something small. Use that moment to push yourself to the very ends of your capability."
She already has her next novel in mind. "I want to write another period novel - early 20th century - and I want to write about girls behaving recklessly. I want to give everybody their chance to do what Alma could only dream of doing. I'm looking forward to writing what I hope will be a very naughty novel."