AceShowbiz - Paul McCartney is glad he recruited poet Paul Muldoon to help him write his new book, "The Lyrics", because their collaboration was like a therapy session the rocker didn't have to pay for.

The Beatles star recalls all The Fab Four songs he wrote in the new tome and he insists writing it with the wordsmith helped him delve deep into the lyrics and really get back to where he was in his life when he penned the tunes.

"He's a great guy and great to work with," Macca tells Costco Connection. "When you talk to someone about the songwriting process, or the stories around it, you find yourself getting very interested. You think, 'Oh, I'd forgotten about that,' or, 'Oh yeah, that's how I did that...' Once I was talking about it, it kind of came flooding back."

"It was just a couple of friends sitting and talking, and the more we got to know each other, the more we could talk about the act of writing... We had a lot of things in common... It was a nice process, actually, and cheaper than a psychiatrist."

Meanwhile, Paul is amazed how much his original song lyrics sell for at auction, confessing if he'd had any idea all those years ago, he would have kept hold of them.

"To my detriment, I would leave a lyric sheet in the recording studio because all I needed it for was just to sing the vocal. I didn't realize it had any value... One day I was walking through the British Museum, and in one of the display cases there was my original lyrics to 'Yesterday'."

McCartney admits he's taking archiving much more seriously now. "I have an amazing archive team and they've really taken the idea on board and found stuff I thought was long lost," he said.