Elvis Costello has mellowed with age. From the spiky vitriol of the classic songs of his "psychotic bank clerk" years, through to the bearded intellectualism of his 1990s collaborations with the Brodsky Quartet and opera singer Anne Sofie von Otter, he has settled into a music to which he seems entirely suited. His first R&B album in nine years, When I Was Cruel, recalls the immediacy of his late-1970s Stiff Records releases, but with an acceptance of life that only age can bring.

"Songs are important to us, most of us, anyway," says Costello of his return to songwriting. "They act as our signposts, date stamps to our life. With this album, I started with the idea of making songs all on my own; I had a toy drum machine and I played over it with an electric guitar. A drum machine is a great way of getting to the start of something new: You can program it to play in a completely unnatural way, like setting up a big-band drum kit and making it play an Egyptian rhythm." Unsurprisingly, Costello's current listening includes classic songwriting, like that of Joni Mitchell. "She's a painter with words," he says. He plays a Joni Mitchell track, Shades of Scarlett Conquering, to illustrate the point. "It's about a woman who models herself on Scarlett O'Hara, and she's terribly overwrought and demanding on all the men in her life: 'Dressed in stolen clothes, she stands cast-iron in furs / With her impossibly gentle hands and long red fingernails.' In two lines you have a complete picture of her. That kind of brevity is the work of a fine novelist."

Brevity is the key, as well, to the songs of Lucinda Williams, another favourite. "On one song, Lonely Girls, she simply repeats the title, then adds an attribute of the lonely girls to each verse."

Nick Lowe, partly responsible for getting Costello started, is also a master of musical expediency. "I definitely listened to Nick Lowe songs and pulled them apart, long before I started doing songs myself. I met him in Liverpool in 1972 at a gig of his at the Cavern, and went up to buy him a drink afterwards. I'd never spoken to someone whose records I had before, and you somehow imagine that they're different animals."

A few years later, Costello was working in early versions of computers, and finding the time to write songs of his own. "You know that Michael Caine film, The Billion Dollar Brain? The computer was just like that: a giant wall of switches and diodes. I had to change the tapes and put paper into the printer. When I realized that the job was a lot easier than my employees thought it was, it became obvious that I was in the perfect position for learning how to write songs.

"This was in the summer of 1976, when it was so hot that all the newspapers were acting as if it was the end of the world, and I was in an air-conditioned office, with all the time in the world needed to just sit and think."

Costello's long-time collaborator Steve Nieve continues to inspire. "You accumulate relationships with producers and other musicians," he says. "Steve has played with my band for years, and I've just spent a year on the road playing guitar and piano for his band, and that experience in itself helped me hugely. He's my friend, and I'm happy that he's found his singing voice."

A recent discovery is Ethiopian pop. "There was a flowering of popular culture in Ethiopia from 1969 to 1978, after the censorship of Selassie's absolute monarchy and before the shutting down by the Marxist regime - the 1970s were Ethiopia's 1960s.

"One man called Ahma put out all the records, recording them in his own studio and pressing them in Greece and India. You can tell the musicians had recently heard James Brown, and apparently one guy had come back from Twickenham of all places and heard jazz, so there are elements of that, too. Most of the musicians were out of the police or the army, as they were the guys who had travelled, and the ones who had been allowed to play under Selassie."

Ethiopian pop is an example of the way in which music talks to itself. "If you take this idea of authenticity too seriously you miss all the fun," says Costello. "Like with blues. One of my favourite bands is the Mississippi Sheiks - it's early blues, but you can hear elements of country in it, because it's music from an experience shared by poor white people as well as poor black people. And then, of course, there's the Rolling Stones. Who would have thought that the music of the black, underprivileged of the American South would be played best by a bunch of callow white boys?"