Why Katy Perry "Will Never Forgive" Russell Brand

Whenever Katy Perry gets around to writing material for her third album, she's got plenty of dark, painful memories to draw from.

As multiple sources confirm in the new Us Weekly, out now, her estranged husband Russell Brand didn't simply blindside her with a Dec. 30 divorce filing; despite public denials from a rep, the British comic, 36, plans to cover all the nitty-gritty, sometimes ugly details of their failed 14-month marriage in an upcoming memoir.

"Katy is insulted and feels betrayed that Russell has treated her so poorly," a pal to the "Firework" singer, 27, tells Us. "She will never forgive him. Ever."

Although Perry is in panic mode regarding her ex's memoir plans, she's back to work on her ongoing concert tour, commercial shoots and more -- and she's taking pains to "limiting…interactions" with Brand, another source says.

"He's being so atrocious that she feels like she never knew him."

For much more on what Brand plans to reveal in his book, how they'll divide their millions in the divorce, how Perry doesn't want to look "battered" and more, pick up the new Us Weekly, on stands Friday.