Joanna Newsom - Make mine a triple, please

Wouldn't it have been great to see the record-label execs' faces when Joanna Newsom, harpist, singer-songwriter and indie pin-up, presented them with Have One on Me, her long-awaited third album? All 18 tracks of it. Over three discs. With an average song duration of seven minutes. In our age of the three-minute MP3, it surely raised some eyebrows.

But people have come to expect this kind of left-field activity from Newsom, one of the most exciting and enigmatic songwriters around. And what a two-hour-and-four-minute musical journey it is, during which you're likely to come across some of the most achingly beautiful and unique songs you'll hear all year. For most critics, Newsom can do no wrong but surely a triple album is a bit much, even for an artist who delights in her quirks as much as she does?

Newsom tells me that she played around with the format before settling on three discs because it "made sense". She spoke with someone at her label about the triple-disc idea "in terms of morning, noon and night, although that wasn't the concept when the album was written but the way that the narrative progresses... it feels a lot like that." When I ask Newsom if the three discs that comprise Have One on Me might be compared to three acts of a play, she ponders, "I haven't heard the description of three acts yet, but I think that makes a lot of sense, actually."

Newsom, 28, was born in Nevada City, California, to doctors who encouraged her and her two siblings to play the piano. The story goes that the six-year-old Newsom instead became enchanted with the harp and begged her parents for lessons. The local tutor decided she would have to wait a couple of years, which little Joanna patiently did before she was finally granted her dream of becoming a harpist by the age of 10.

She continued to play throughout her school years before enrolling at Mills College in Oakland, California, with the intention of becoming a composer. After being disappointed with the course, she dropped out and began making some rough recordings of her songs. One of her CDs made its way to the anti-folk hero Will Oldham (or Bonnie "Prince" Billy, as he is better known), who invited Newsom to accompany him on his US tour. She eventually signed to Oldham's label, Drag City, and released her debut album, The Milk-Eyed Mender, to great acclaim in 2004. Ys followed in 2006 and cemented her status as a folk sensation.

Just as there was a marked difference between The Milk-Eyed Mender and Ys, her sound has evolved yet again for Have One on Me. The most apparent change is that her voice is barely recognisable. It is much more restrained and natural-sounding than it has been in previous releases. At times she sounds like Joni Mitchell, at others like Kate Bush. It's a pleasing aural aesthetic that might ingratiate her with those who had previously deemed her too shrill or witchy.

That's not to say that it was a pre-meditated move. "The process of touring constantly, definitely changed the voice, often in ways that I wasn't totally conscious of," she says. Then, after two years of touring, disaster struck. "I ended up injuring myself and had to go on voice-rest for a few months and so, of course, at the end of that, all of my singing ability had just atrophied." She saw doctors, speech therapists and eventually, a vocal coach. She concedes, "I'm not totally sure what happened but it definitely feels a lot different now."

While the harp remains the primary instrument on the record, the sound is more accessible. Newsom admits that the "harmonic ideas are a lot less abstract" and it's "a lot more spare" than the dense arrangements of Ys. The full orchestra has been replaced by folksy instruments such as the banjo, oboe and mandolin. She points out that the presence of drums also "helps somehow" in making it more grounded. However, it's still an epic album, sounding at one point like it was recorded in a bar in the Wild West and at other times in a Tudor court.

But that is not to say it doesn't make sense. Themes of love, home and debauchery recur throughout. The title, Have One on Me, was chosen for its association with drinking and decadence as well as its emotional connotations. She explains, "It felt kind of appropriate to have the expression people use when they're buying a round, but also in the song that the title comes from, the expression is sort of used as an example of self-sacrifice. I was thinking of it in terms of a bottle that's full, and then you pour into someone else's glass, and it's a little less full, and the giving of yourself diminishes the self."

Elsewhere on the album, notions of romantic love, family love and friendship are explored. "Good Intentions Paving Company" is a rollicking, brilliant song that compares a journey being made across country to the journey of a relationship. The album ends with the heartbreaking "Does Not Suffice", which sees her packing up all her belongings so that "everywhere I tried to love you/ is yours again/ and only yours". The "tap of hangers, swaying in the closet" says it all.

"'81" is a gorgeous little track, presumably about her conception, and it's a classic example of Newsom's use of nature in her work: "I found a little plot of land/ in the garden of Eden./ It was dirt, and dirt is all the same," she sings.

In the past, Newsom has found it difficult to disassociate herself from the woodland figure of her songs. There's a degree of self-mythologising in her music, with its references to courtyards, Mother Nature and the earth, such that fans don't want to accept that she doesn't live in a tree, and actually exists in the 21st century. She tells me she's "conscious of the possibility of disappointing someone when they speak to me, that I'm not going to meet expectations, because that's not who I am".

She is after all, just a girl. A girl who likes fashion (she has modelled for Armani and tells me excitedly that the sisters behind the avant-garde label Rodarte are "in their own amazing artistic world"); one who dates the Saturday Night Live comedian Andy Samberg (probably most famous over here for his "Dick in a Box" comedy skit with Justin Timberlake); a girl who likes to "go out to eat with my friends and family", and listen to Dirty Projectors and Roy Harper. One who loves Hemingway and Nabokov. I ask her when she would live if she could live at any point in history, and she responds with, "Now. Twenties Paris I wouldn't have minded, or 1970s southern California, but I wouldn't have been able to know the people I know. The main things that matter to me are relationships and friends and family."

Further eschewing the idea that she exists in some other-worldly realm, Newsom has licensed some of her tracks to be used in adverts for HSBC and Victoria's Secret. "It might be distasteful but it's a totally necessary thing. That's what allowed me to survive while I didn't tour for two years, and make this record. I wasn't living off royalties, that's for sure," she giggles.

Even though the album is still very earthy, the references to decadence may suggest that Newsom is trying to escape the nature-child vision fans have of her. The colourful artwork for Have One on Me sees her lying on a sofa, heavy of make-up and surrounded by an Old Curiosity Shop of opulence: peacocks, fawns, leopard print, silks. When I ask her how involved she was with creating the over-the-top scene, she explodes with laughter: "Those are all my things! It's pretty insane when it's altogether. I wanted to evoke the Edwardian era and the roaring Twenties... We had all these touchstones and all of it was meant to connect back to the way the album feels, and that decadence."

Newsom is so sweet that you can't imagine her being debauched. I ask her, if she was to have one on me, what would her tipple be? "Oh, it definitely depends on what's happening! Probably some sort of whiskey. More often, I just get a beer." And there you have it. Joanna Newsom: her music may be celestial, but, at her insistence, she's just a regular beer-drinking girl.

'Have One on Me' is out now (Drag City). Joanna Newsom plays the Royal Festival Hall, London SE1 (0844 875 0073) 11 & 12 May