There was a six-year gap between Heart's latest album, "Red Velvet Car" in 2010, and its predecessor, "Jupiter's Darling." But Ann and Nancy Wilson aren't planning to take nearly as long again.

In fact, Nancy tells Billboard.com, they're already well along on the next one.

"We've got most of it written already," Wilson says. "We're writing again and working on songs with ('Red Velvet Car') producer Ben Mink. This time we're gonna go a little more heavy rock, a little more electric guitar. And so far so good; I think there's some really amazing songs already."

Wilson says the sisters and Mink have demoed "about eight songs already," working "in various hotel rooms...with microphones and his laptop and a couple of speakers and some guitars and notebooks. Me and Ann work on stuff independently of each other and we bring our notebooks, and (Mink) brings musical ideas, too, and we write together and just put everything together."

Heart -- which this year celebrates the 35th anniversary of its debut album, "Dreamboat Annie" -- hasn't previewed any of the new songs yet during its summer tour with Def Leppard, but Wilson expects to play something new before the trek wraps Oct. 29 in Australia. "It's not unlikely that we would probably road test something in the course of the summer," she predicts. "We're still kind of polishing and buffing the rough demos so far, but we're getting excited."

Heart expects to hit the studio "as soon as the tour is done...and hopefully we can have everything recorded by Christmas."

Meanwhile, the Wilson have joined forces with Seattle-based writer Charles Cross -- who's authored books about Kurt Cobain, Led Zeppelin and Jimi Hendrix -- on a memoir they hope to publish in 2012. "We're telling the whole story," Wilson says. "I think it's important to be kind of brutally honest without making anyone else feel bad in any way, if possible. There's a lot of stuff the you remember that's like, 'Wow, why did I do this with that person?' or 'What was I thinking?!' or 'How controlling was so and so?' It's definitely not a sensational, tell-tale book because that's never who we are. There's going to be some wild parties and some failed relationships, but not too much more dark stuff really."

Wilson adds that she's been particularly surprised by sister Ann's "incredible" memory during the process. "I feel like I've had a lot of painful situations that I intentionally delete from my memory," says the guitarist, who divorced from husband Cameron Crowe in 2010. "So it's been really good to have Ann there to remember a lot of this stuff because I think I kind of have defense mechanisms with forgetfulness."