“Welcome to the “House of Hilton,” Jerry Oppenheimer’s thorough new tell-all that tries to explain the familial follies that unleashed the famous-for-nothing Paris Hilton on our world.

“Every generation has an It girl and Paris became this generation’s It girl,” says Oppenheimer. “My goal in doing this book was to figure out how she became who she is.”

“I came away from this research with a lot of sympathy for Paris. I came away feeling a bit sympathetic for her because I don’t think she had a chance to do anything besides what she is doing.”

What really knocked Oppenheimer’s socks off, he says, was the discovery of Paris’ maternal grandmother, Kathy Avanzino Richards, a woman from Manhasset everyone called Big Kathy, who truly was larger than life.

“I don’t think anyone has ever really talked about that before,” Oppenheimer says.

But the Hilton side of the family had its share of skeletons, from Paris’ great-grandfather Conrad, who used to take his second wife, Zsa Zsa Gabor, to houses of ill repute while he slept with other women to Paris’ dad Rick, who was never really given a slice of the Hilton hotel fortune.

“While in name she is a Hilton, she is not the heiress the world has been led to believe,” Oppenheimer explains. “In fact she has to work her tush off to live the Hilton lifestyle. The bulk of the Hilton fortune is going into the Hilton Foundation. Paris is one of many grandchildren and great-grandchildren who have to work for a living.”

Nicky Hilton

* Two years younger than Paris, Nicky has somehow managed to avoid the level of scandal of her older sister.

* Childhood friends were wary of Nicky because of her family. “Nicky wasn’t included in a lot because she was rich, because she had the Hilton name — you know, she was the first kid to bring a cell phone to school, so all of that made it difficult, and Paris already had a reputation at the school,” remembers Gini Tangalakis, the mother of one of Nicky’s childhood friends. “It didn’t appear to me that there was much parental involvement.”

Big Kathy

* Paris Hilton’s maternal grandmother, “Big Kathy” Avanzino Richards, was the real spark plug of the Hilton women. “One of the things with Big Kathy that shocked me was the violent nature this woman had. I would come away from the interviews with my jaw dropped. She was the original ‘Bad Seed,’ ” Oppenheimer says.

* Paris must have learned her manners from Big Kathy. A close childhood friend of Kathy’s, Jane Hallaren, remembers Kathy pointing at another little girl’s beautiful brown curls, and then, “I look and Kathy had spit all over this girl’s hair. I could see gobs of spit. And Kathy was giggling. I still think of the scene and I get the willies,” Hallaren told Oppenheimer.

* Big Kathy’s first daughter, Kathy (Paris’ mother), was conceived in the back seat of a 1957 black Chevrolet convertible with boyfriend Larry Avanzino. The couple then had a shotgun wedding to please their families. It would be the first of four for her.

* Big Kathy once bragged to a friend, “my daughters are married to men who have a total net worth of $13 billion.”

* She once put a screw in a cheeseburger meant for her stepdaughter Diane, Oppenheimer writes, because she thought Diane was undermining her marriage to her second husband, Ken Richards. Another time, she fed her mother-in-law dog food and then laughed about it.

* Desperate to marry her daughter off to Rick Hilton, Big Kathy helped convince her future son-in-law that Kathy Jr. was a virgin before he married her.

Kathy and Rick Hilton

* Rick Hilton’s claim to fame at the University of Denver was playing volleyball and hosting hotel parties. He was once the target of a staged kidnapping by fellow students.

* As newlyweds, Kathy and Rick enjoyed pushing their weight around at various Hilton hotels across the country, especially Las Vegas, where Kathy (like her daughter) loved to party. They’d bring young daughter Paris out already painted up in mascara and eyeliner.

* During one incident at a Hilton hotel, Rick and Kathy left their dogs alone in a suite for three days, without walking them. “There was dog shit and dog piss all over the place,” a source told Oppenheimer.

Paris Hilton

* Paris was once attacked by her pet monkey, but the heiress has always had a thing for small animals. When she was 12, she carried her pet ferrets around in her Prada purse, causing headaches for Hilton hotel managers, who were terrified to reprimand her because they feared Kathy’s wrath.

* At Hilton hotels, Paris demanded the same VIP treatment that her parents received. One insider told Oppenheimer she “used the saunas in one hotel to go to the bathroom because they were too lazy to get out of the sauna to use the actual bathroom.”

* Though Kathy Hilton explained her daughter’s disappearance from New York prep school Dwight Academy as the result of a stalker in the Waldorf-Astoria, which supposedly forced the girl to leave town, managers at the swank hotel where the clan lived remember that Paris disappeared because she ran off and had a fling with a cute trucker from the hotel’s loading dock.

* “I have family sources telling me that Rick and Kathy Hilton are proud of everything Paris has done, including the X-rated video that launched her to this international stardom and infamy and fame,” Oppenheimer says.”