When Def Leppard guitarist Phil Collen thought about having guests on the self-titled debut album by his blues side project Delta Deep, he came up with two appropriate names -- bandmate Joe Elliott and David Coverdale of Whitesnake and Deep Purple fame. The collaborations just took a different turn than he planned.

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His initial idea was to have Coverdale cover "Mistreated," which he sang on 1974's Burn -- his first album with Deep Purple. But Coverdale was making his Purple Album of Deep Purple covers with Whitesnake and planned to include "Mistreated" on that, though he agreed to sing on a version of William Bell's "Private Number." Then, Collen reports, "David also said, 'I'd love to hear Joe singing 'Mistreated.' I said, 'Me, too.' I see Joe pretty much every day a lot of the time, so I just asked him and he said, 'I'd love to,' especially because he knew it inside-out. And a lot of people say, 'It's the best performance I've heard Joe do.' I love it as well. It was perfect."

Listen to Elliott wailing on Delta Deep's cove of "Mistreated," which Billboard is premiering exclusively below.

Collen formed Delta Deep in 2012 with singer Debbi Blackwell-Cook, godmother of his wife Helen. They eventually filled out the lineup with Stone Temple Pilots bassist Robert DeLeo and drummer Forrest Robinson (India.Arie, TLC, the Crusaders and more), though members of Collen's other side project Man-Raze also did some recording for the Delta Deep album, which comes out June 23.

"It's very different from Def Leppard," Collen explains. "This is actually a more expressive kind of music. With Def Leppard there's a routine and technique to it, even the songwriting and stuff; you can't just jam off and do stuff like that. You're part of a very specific team where everyone has to do their bit, and it's a lot more planned out -- otherwise it doesn't work. (Delta Deep) is way more expressive and it's more of a performance. Even my guitar playing is different; I just blast away and have fun, and it's totally free."

Delta Deep is hardly Collen's only solo endeavor at the moment. On Oct. 27, he'll publish a memoir, Adrenalized: Life, Def Leppard and Beyond, that he co-wrote with journalist Chris Epting. "It's wide open," Collen says of the book. "It's my story type of thing. It's pretty interesting things that happened along the way, from growing up in London to the travel and meeting different types of people and how that changes your life -- the crazy highs, the lows everyone has, just the whole thing, really, and how it changes your outlook on life and your perception." Collen adds that readers will probably be surprised to discover that his time in Girl, the band prior to Def Leppard, was "a lot crazier and darker and nastier" than what transpired even during Def Lep's heyday.

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Def Lep, meanwhile, returns to the road June 20 for a summer North American tour, which will include shows with Styx and Tesla. The group is also finishing up a new album that Collen predicts will be out this fall and features one track on which all five band members sing a bit of lead vocal.

"It's really diverse," Collen says of the follow-up to 2008's Songs From the Sparkle Lounge. "I honestly think it's the best thing we've done probably in about 20 years. We went in to do a single or an EP and we came out with 12 songs. There was absolutely no pressure. There was no record company executive. There was no record label. We did it for ourselves, and the songs range from flat-out hard rock, almost punky on some of the stuff, to classic Leppard, and there's some other tracks that sound like they'd be on top 40 radio -- just the massive choir of vocals and melodies and all this stuff going on. We treated each song as a different project and did it over a year and a half, which really did help it. I think people are going to like it a lot."