"I didn't realize how close I was to checking out," Cole, 58, said Thursday.

It was a dramatic downturn that Cole, her doctors and those around her didn't expected. She had been on the tail end of treatment for hepatitis C, which she learned she was suffering from last February. It was a consequence of her much-publicized drug use in the 1970s and '80s -- for years she was addicted to heroin before she successfully completed rehab in 1983.

Hepatitis C typically has a decades-long dormancy period, and before Cole received the diagnosis earlier this year, she'd felt no ill effects. After starting Interferon treatments in May, though, Cole said the effect was "debilitating. [It] was worse than the disease."

Cole will take her first steps back into the public spotlight today with an autograph session at the Borders store in Westwood and an appearance Saturday at the Pottery Barn store at Costa Mesa's South Coast Plaza shopping mall. Next month, she slowly will resume the concert schedule she would have mounted to support her latest release, "Still Unforgettable," had everything gone according to plan.

The album was conceived as the successor to Cole's 1991 multiple Grammy winner "Unforgettable: With Love," which became the biggest hit of her career -- selling more than 6 million copies in the U.S., according to Nielsen SoundScan. Cole wowed audiences with a seamless duet with her late father's voice on the title tune, one of Nat's signature numbers.

On "Still Unforgettable," the likes of Alan Broadbent, Bill Holman and others give lush arrangements to another group of American pop standards, and again, Cole duets with her father's voice on the gently swinging "Walking My Baby Back Home."

Her Interferon treatments made the recording process much more difficult, as some of the side effects -- lack of appetite, diminished thirst -- rendered her dehydrated and increasingly weak.

She still doesn't know whether the hepatitis treatment contributed to her failing kidneys, but she's now on dialysis three hours a day, three days a week "for as long as I can hold out." The alternative: a kidney transplant.

For now, she's feeling considerably better, and she welcomes the improvement.

"I couldn't get up for three weeks -- I couldn't even lift my head off the pillow," she said, her green eyes still regaining their sparkle. "I was sleeping all the time."

Her hair is cropped short, and she looks thin, not gaunt. "I could still use about another five pounds," she says with a soft smile.

"Still Unforgettable" has sold 57,000 copies in five weeks. That's almost what her previous studio album, "Leavin'," has sold in two years, but still below what it might have done in a perfect world. So she's all the more antsy to get out and perform the new songs with a big band orchestra.

"The thing so disheartening to me was how long she had waited to put this record out," said Tena Clark, a longtime friend of Cole's, executive producer of the new album and president of Pasadena-based DMI Records, which released it. "It took her a long time, but she finally felt like she was in the right place to do it. . . . Sure, we took a hit, because there was about a month and a half where she was silent and not doing any press. Obviously that's not a good thing for a record," Clark said. "But honestly, I never lost any sleep over that part. The money didn't matter, I just wanted Natalie to be well and get back on her feet . . . The great thing about this record is that she's created a very timeless piece."

Cole's hoping to be strong enough to set out on a formal tour next year. It will be more challenging than a standard tour because she'll need to undergo dialysis in various cities while she's on the road.

"I'm never sick," she said. "I'm the type of person who won't cancel a show even if I don't feel my best."

Said Clark: "She really was a trouper . . . I'll never forget it -- it's just amazing the stamina and the pure will that some people have. And Natalie is one of them."

But she's got more on her mind than just promoting her own record or even her own career. She's also looking out for the health of the Great American Songbook.

"We can't lose this music," Cole said. "That's why I want to get back out there in the world."

Lewis is a Times staff writer.

randy.lewis@latimes.com