In a series of tweets early Saturday, President Donald Trump slammed author Michael Wolff and described himself as "a very stable genius." He also tweeted about "Russian collusion," the "Fake News Mainstream Media" and the unemployment rate for African-Americans, among other topics.

"Michael Wolff is a total loser who made up stories in order to sell this really boring and untruthful book," Mr. Trump tweeted, seemingly referring to "Fire and Fury," Wolff's bombshell expose on the Trump administration. "He used Sloppy Steve Bannon, who cried when he got fired and begged for his job. Now Sloppy Steve has been dumped like a dog by almost everyone. Too bad!"

Michael Wolff is a total loser who made up stories in order to sell this really boring and untruthful book. He used Sloppy Steve Bannon, who cried when he got fired and begged for his job. Now Sloppy Steve has been dumped like a dog by almost everyone. Too bad! https://t.co/mEeUhk5ZV9 — Donald J. Trump (@realDonaldTrump) January 6, 2018

It's his latest pushback against the book that portrays him as a leader who doesn't understand the weight of the presidency. In the book, former aide Steve Bannon questions Mr. Trump's competence.

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Mr. Trump said "the Democrats and their lapdogs, the Fake News Mainstream Media, are taking out the old Ronald Reagan playbook and screaming mental stability and intelligence....."

Now that Russian collusion, after one year of intense study, has proven to be a total hoax on the American public, the Democrats and their lapdogs, the Fake News Mainstream Media, are taking out the old Ronald Reagan playbook and screaming mental stability and intelligence..... — Donald J. Trump (@realDonaldTrump) January 6, 2018

"Throughout my life, my two greatest assets have been mental stability and being, like, really smart," Mr. Trump tweeted. "Crooked Hillary Clinton also played these cards very hard and, as everyone knows, went down in flames. I went from VERY successful businessman, to top T.V. Star ... to President of the United States (on my first try). I think that would qualify as not smart, but genius....and a very stable genius at that!"

....Actually, throughout my life, my two greatest assets have been mental stability and being, like, really smart. Crooked Hillary Clinton also played these cards very hard and, as everyone knows, went down in flames. I went from VERY successful businessman, to top T.V. Star..... — Donald J. Trump (@realDonaldTrump) January 6, 2018

....to President of the United States (on my first try). I think that would qualify as not smart, but genius....and a very stable genius at that! — Donald J. Trump (@realDonaldTrump) January 6, 2018

At Camp David later on Saturday, reporters asked Mr. Trump why he felt the need to take to Twitter to defend his intelligence.

"Well, only because I went to the best colleges or college," the president said. "I went to – I had a situation where I was a very excellent student and came out and made billions and billions of dollars became one of the top business people. Went to television and for ten years was a tremendous success as you probably have heard. Ran for president one time and won."

"And then I hear this guy that does not know me – doesn't know me at all," he continued. "By the way, did not interview me for three – said he interviewed me for three hours in the White House. It didn't exist, OK? It's in his imagination. And what I was heartened by because I talk about fake news and the fake news media – was really that so many of the people that I talk about in terms of fake news actually came to the defense of this great administration and even myself because they know the author and they know he's a fraud."

Mr. Trump also discussed a range of other topics while speaking to reporters Saturday in the middle of his retreat with key congressional leaders and senior administration officials. He said he's had some "incredible meetings" with Republican congressional leaders and Cabinet members at Camp David as they ready their 2018 legislative agenda.

He said the group discussed a variety of topics, from national security and infrastructure to the military and the budget.

"We are very well prepared for the coming year. We finished very strong," he said.

Mr. Trump is hoping to work out a deal with Democrats that may include granting legal status to about 800,000 young immigrants who were temporarily shielded from deportation under an Obama-era program known as Deferred Action for Childhood Arrivals. He said "it's something certainly that I'd like to see happen." Mr. Trump also wants to address the problem of drug "pouring into this country and how to stop it."

On the topic of North Korea, he said he would "absolutely" engage in phone conversations with North Korean leader Kim Jong Un.

Early Saturday, Mr. Trump tweeted about the unemployment rate for African-Americans, which in December was at the lowest level since the government started tracking such data in 1972. The reasons for the record low range from a greater number of black Americans with college degrees to a growing need for employers in a tight job market to widen the pool of people they hire from.

"The African American unemployment rate fell to 6.8%, the lowest rate in 45 years. I am so happy about this News!" Mr. Trump said.