Elvis Presley’s fiancée Ginger Alden details the King’s bad temper, secluded …

Elvis Presley promised Ginger Alden “the wedding of the century” before they went to sleep at Graceland on that hot August night in 1977. The next time she saw him, he was dead on the bathroom floor.

“Elvis Ginger: Elvis Presley’s Fiancée and Last Love Finally Tells Her Story” is Alden’s anticipated new memoir — in which she at long last reveals the intimate details of the King’s strange courtship down to its terrifying moments.

Alden was 20, living at home with her parents in their modest Memphis home when her sister Terry, the then-reigning Miss Tennessee, got a call from Graceland. It was late on a Saturday night in November, but the King wanted to know if she could come over. Terry asked if she could bring her two sisters.

The women were given a tour of the ground floor of Graceland while they waited several hours for Elvis to make his entrance. Ironically, it was one of the few times that Alden ever saw the downstairs. Though she would soon all but live at the famed mansion, her life with Elvis was confined to the upstairs bedroom suite.

Elvis, 41, finally appeared and was immediately taken with Alden. He read to her that night from Khalil Gibran’s “The Prophet.” They would spend many nights like that, Elvis reading aloud from his texts of Eastern spiritualism or books on numerology.

She first joined him on tour in Las Vegas, where he sent her to luxury stores in the hotel lobby to buy expensive gowns to wear to his shows. Then came the jewelry. Just before the show one night, he asked her to close her eyes. When she opened them, there was a gold diamond cluster ring on her finger.

At almost the same moment, he placed a ring of sapphires and diamonds on her other hand, announcing, “You have to have backups.” Elvis produced two more diamond rings. A couple of days later, he gifted her with a new Lincoln Mark V.

But she was also treated to her first taste of the King’s temper. When she refused to immediately break up by phone with a man she had been seeing, Elvis slammed into the next room, where his entourage sat. With an audience, he hurled a bottle of Gatorade against the wall.

It was that night that they first made love. Elvis opened her robe, but refused to strip off all of her lingerie. His pajamas didn’t come entirely off, either. “I don’t believe people should be completely undressed until they’re married,” he told her before consummating their relationship.

Before his next show, he gave her another diamond necklace and a diamond watch.

Alden noticed that while they were in Vegas, she saw nothing beyond the hotel suite and the showroom. Elvis didn’t get out much.

Back at Graceland, he spent most of his days and nights upstairs in his pajamas, rarely venturing beyond his bedroom or adjoining office. In their time together, she never saw him eat at a table. Meals were delivered to them in bed.

To spruce up, Elvis might throw on a jeweled blue bathrobe. Or if there was a need to leave the house, he would pull a jumpsuit over his pajamas and strap on a belt to accommodate the gun he always carried. Among his many firearms, Elvis owned several magnums.

One night, when she couldn’t stay awake to keep reading with him, she saw one of the magnums in action — or rather heard it. A deafening roar woke her up. Elvis had fired the pistol at the wall over the headboard.

He explained that he had asked her to get him some more yogurt and she hadn’t jumped to. “It was an attention-getter,” he explained.

Elvis and guns were a dangerous combination. One night, when the toilet started to gurgle, he blasted it with a machine gun. More frightening was the day he raced into the yard with the machine gun after spotting his daughter, Lisa, being pursued by someone with a gun. Alden made him see that Lisa was being chased by a child with a toy pistol.

He once shot the television when he didn’t like the program. Another time he fired at the phone when it disturbed him. Alden blamed such erratic moments on his mood swings.

She was always worried by the “sleep packets” delivered nightly by the nurse who lived in a trailer out back. Often, Elvis would need to be dosed more than once throughout the night. But if she said anything, he would fly into a rage.

She couldn’t even risk reproaching him about the enormous quantities of food he would gorge on. One night, on vacation in Hawaii, she tried to reason with him that he had consumed so much papaya juice that day he really didn’t need any more.

Furious, he announced, “We’re leaving Hawaii because of you,” threatening to take her, her family and the huge entourage that had accompanied them home. When she walked out on his rant, he stormed into the room and slapped her across the ribs.

“No one ever walks out on me when I’m talking,” he yelled.

Still, there were loving, generous gestures. He loaded Alden and her family with mink coats. Her parents drove a new car. He insisted on taking over the mortgage note on the family home when her parents refused to let him buy a new house. And always there was more jewelry, including many diamond-studded pieces.

Barely three months after they had met, Elvis seated Alden in a chair in his bathroom, dropped to his knee and presented her with a huge diamond ring, a center-cut stone surrounded by six smaller diamonds.

“Ginger, I’m asking you. Will you marry me?”

They kept the engagement quiet, not wanting to surprise Lisa with the news until plans were formalized. He repeatedly told Alden that God would let him know when the time was right to wed. It was on a summer night that he set the date. They would marry that Christmas. It would be “the wedding of the century,” he said.

“I’ve thought about your gown. The dress should have a high collar and I would like it to have small rosebuds with gold threads through it. I’m gonna have someone work on it in Los Angeles,” he told her.

The next day, she got up about 2:20 p.m. Elvis wasn’t in the room but she noticed the door to his bathroom was cracked open. She relates what she saw.

“Elvis looked as if his entire body had completely frozen in a seated position while using the commode and then had fallen forward, in that fixed position directly in front of it.

“His legs were bent, the upper part of his chest and shoulders touched the ground, and his head was slightly turned to the left.”

The commotion was huge — little Lisa had to be blocked from the bathroom door — and there were attempts to revive him. But the King was officially pronounced dead, a victim of cardiac arrhythmia, at Baptist Memorial Hospital in Memphis on Aug. 16, 1977.

At the funeral, Alden took a backseat, though Priscilla Presley, Elvis’ ex-wife, whom Alden resembles, told her at the wake, “I know how much Elvis loved you.”

Soon after, Alden’s mother got a notice that the mortgage had not been paid. Even a lawsuit didn’t move the estate to fulfill Elvis’ promise. Alden was left with her box of jewels and memories that she has refused to share until now.

“Elvis Ginger” is on sale Sept. 2.