A new tell-all out today has Larry Birkhead and Howard K. Stern up in arms!

A shocking new book about the men surrounding the late bombshell ANNA NICOLE SMITH alleges that romantic rivals LARRY BIRKHEAD and HOWARD K. STERN were actually lovers!

Both men deny the claims and have told us they will sue! Now, the author of the book, former MSNBC reporter RITA COSBY, is defending her accusations in a new interview.

"These men had a relationship," she tells "The Insider"'s PAT O'BRIEN.

Rita says some of her sources for Blonde Ambition: The Untold Story Behind Anna Nicole's Death include Anna's one-time friend, former pro-volleyball player JACKIE HATTEN, whose brother is currently serving time behind bars for threatening Anna. (However, others say that Jackie had not seen Anna since well before her brother was convicted.)

Rita also claims that there is even a mysterious videotape that proves the two men were involved, although she says, "I personally did not see the videotape."

"I stand by the reporting in my book," Rita says.

But Howard's attorney LIN WOOD says, "Miss Cosby's book has taken the coverage to a disgusting new low for which she will be held legally accountable."