Hillary Clinton was a White House monster when she was first lady.

That’s the assertion in a new book called “Crisis of Character” by Gary Byrne, which hits stores Tuesday.

One of Hillary the Monster’s favorite targets was Vince Foster, according to Byrne, a member of the Secret Service who was stationed in the White House during the Clinton administration.

Foster was the lawyer who relocated to Washington from Arkansas after his childhood pal, Bill Clinton, was elected president.

“Word circulated that (Hillary Clinton) berated (Foster) mercilessly … I knew what it was like to be yelled at by superiors, but Mrs. Clinton never hesitated to launch a tirade,” wrote Byrne.

Byrne doesn’t know the half of it — which I’m now going to tell you.

Foster committed suicide in McLean, Va.’s, Fort Marcy Park on July 20, 1993. The 48-year-old shot himself in the head. Friends later reported that he was very — maybe even clinically — depressed.

At the time, there were suspicions of murder and coverups. Foster had, after all, been involved in many of the Clintons’ questionable financial transactions, both as their lawyer and as a participant in those deals. And those deals were getting increasingly embarrassing for the Clintons as well as others in Arkansas and Washington.

All of the deals came to be known by the name of just one — Whitewater.

I was investigating the Clintons back then for The Post because this started out mainly as a financial story — a big one. And toward the end of the 1990s, I got to see a large number of documents from the Whitewater investigation, most of which have not been seen by outsiders.

So now I’m going to add to Byrne’s stories about Hillary. Most of this stuff has, until now, been overlooked because the focus has been on Bill Clinton and his sexual shenanigans back then.

Some of you will argue that this is ancient history and shouldn’t be dredged up. But whenever someone runs for the highest office in the country, their whole life becomes relevant.

Hillary has rightly raised some ancient bankruptcies of Donald Trump, her likely Republican foe. It’s relevant.

Before they got to the White House, Hillary and Foster were good friends. Some suspected they were even more than that, evidenced by the fact that Foster was caught slapping Hillary’s backside at a birthday party one night at a Little Rock bar.

That story was told to me more than 15 years ago by Arkansas state troopers who witnessed it and thought the two were having an affair, although they didn’t have any hard evidence.

Lisa Foster, Vince’s widow, was even asked by an investigator about the relationship between Hillary and her late husband but she refused to discuss it. The investigator thought that was suspicious, but Lisa threatened to stop talking unless the subject was changed. “Her face got red and she was a little upset,” said the investigator of Foster.

So it must have been a shock to Vince Foster when he got to the White House in an exalted position, thrilled that his old friend had won the presidency and his good pal, Hillary, was also in a position of power — only to be put down by her at every turn.

Lisa Foster told the investigator that Vince “couldn’t laugh, couldn’t smile” during his time at the White House. “He was from a law firm [the Rose Law firm] where they would handle one thing at a time. Suddenly, he’s got 40 things going on at one time,” the investigator told me.

Among those multiple things on his plate were probes of the Clintons’ firing of workers in the White House travel office, the Whitewater transaction, other financial transactions of the Clintons and likely testimony before Congress.

Vince Foster even made an appointment with a psychiatrist shortly before he died, but he killed himself before that meeting.

A suicide note was found — but not until six days after he died. And it was suspiciously located in his briefcase, torn to pieces with one piece missing. The briefcase was supposedly searched right after his death and — again presumably — no suicide note was discovered.

The contents of the note were never released and Lisa refused to discuss the matter with an investigator. There were rumors that it wasn’t even a suicide note — that it was just the ramblings of a disgruntled White House insider written well before he took his life.

People in Whitewater special prosecutor Ken Starr’s office had a lot of questions about the note.

Specifically, they wanted to know why Bill Clinton was told about the note so much later than Hillary was told. Bernie Nussbaum, counsel to the president, told Hillary about the note and “she had said he should handle it and was emotional,” according to a previously undisclosed document from Starr’s office.

“Nussbaum did not say whether he had actually shown HRC [Hillary Rodham Clinton] the note,” according to the document.

On July 27 — seven days after Foster’s suicide — there was a meeting about the note. Lisa Foster was heading to Washington later that day and “[White House chief of staff Mack] McLarty was present when Lisa came to the White House and saw the note. She was more composed than he expected,” according to the document.

President Clinton was apparently busy that day and still hadn’t been told of the note. McLarty eventually told him. “Mack does not believe the president knew about the note before he told him,” the Starr document said. McLarty was asked: “Does it puzzle you that the first lady had not told the president about the note? and Mack said, ‘No.’ ”

Hillary has a lot of secrets and others will be coming out over the next few months. As I’ve been saying, the Democrats should have picked a better candidate.