The girls are fighting ... because Paris Hilton blew off being interviewed for Kim Kardashian's biography a whopping 17 times.

"Citizen Kim" author Jonathan Van Meter told The Hollywood Reporter that Paris repeatedly bailed on him when he tried to interview her for Kim's authorized biography.

He told the outlet he had to go through Paris' team, whereas for other celebs he says all Kim had to do was send a text.

Jonathan said ... "I sent questions, connected with Paris' assistant, and then the interview was canceled 17 times over about four months."

After enough cancellations, he actually found the incident "more interesting" than anything Paris had to actually say.

He says Kim was "so mad" when he told her what happened, saying ... "It seemed to stir up some ancient irritation between them. The creation turning on the creator."

It's unclear why Paris kept cancelling ... maybe Jonathan's got another book on his hands.