Then-candidate Donald Trump signs a book during a commercial break at the Fox Business Network Republican presidential debate. Scott Olson/Getty Images During his time in the White House, President Barack Obama engaged in a ritual highly scrutinized by the publishing community and book-lovers: The release of his summer vacation reading list, usually a mix of critically-acclaimed fiction and historical non-fiction.

But while August came and went without a list of big-name novels that President Donald Trump could've read in between rounds at his New Jersey golf club, Americans didn't need to wait until summer for him to share his recommended reads.

On several different occasions this year, Trump has used his Twitter account to promote nonfiction books with distinctly political bents by supporters who have advocated for his agenda on television.

Though one endorsement was sarcastic — Trump advised Americans to buy "Reasons To Vote For Democrats," a book that has 260 blank pages — the president has used his Twitter account on four occasions during his presidency to promote books by vocal advocates for his candidacy and administration.

Last week, Trump recommended Americans pick up a book by Sheriff David Clark, a highly-polarizing Wisconsin law enforcement official known for his brutal jails, who frequently praises the president and spoke on Trump's behalf at the 2016 Republican National Convention. He was also offered a job in the administration.

In March, Trump described Fox News guest Nick Adams' 2016 screed against a US consular officer as a "must read." Adams believed the office wouldn't let him into the country because he is a conservative.

And on at least one occasion, Trump's blessing came right at the perfect time: On book release day.

In a brief interview with Business Insider in June, currently suspended Fox News host Eric Bolling said that he'd given the president a manuscript to his new book months before its release, during a visit to the White House.

Although he said he didn't know whether Trump read the book, Bolling strongly hinted that when his book was released the next day, the president may give it a bump.

"Who knows? Maybe he'll tweet about it," Bolling said at the time. "Keep your eyes on Twitter."

Early the next morning, Trump did indeed retweet Bolling's promotion of his book.

Other right-wing pundits have also floated around the White House and nearby Eisenhower Executive Office building, plugging their books with members of the administration.

Conservative author and documentarian Dinesh D'Souza visited the White House earlier this month and posed for a series of photographs holding his new book "The Big Lie: Exposing the Nazi Roots of the American Left" with since-departed White House staffers Steve Bannon and Sebastian Gorka.

Though he detailed the tweets just minutes after posting, D'Souza said he briefed both Bannon and Gorka on the contents of the book, which makes the convoluted argument that Adolf Hitler's ideas indict the modern day Democratic Party.

Ethics experts told Business Insider that while Trump is allowed to promote books, other recent presidents have steered clear of explicitly telling Americans to read books with overtly political agendas. Obama's list was never an official endorsement, though a spot on the president's reading list usually corresponded with a bump on Amazon.

"Legal but tacky," Richard Painter, the chief ethics lawyer for former President George W. Bush, said in an email describing Trump's promotion of political books. "And without precedent that I know of."

Government ethics experts have repeatedly raised hackles over the Trump White House's continued promotion of his family's businesses while in office.

In March, the Office of Government Ethics criticized the White House for its refusal to discipline Trump counselor Kellyanne Conway, who promoted Ivanka Trump's products on air during an interview on Fox News. And since-departed OGE chief Walter Shaub has repeatedly admonished Trump for visiting his own properties, where he has made appearances at weddings.

Despite concerns over the great monetary benefits the Trump family could reap from the promotional power of the bully pulpit, the opposite has happened on some occasions.

At least 17 charities have pulled events from Mar-a-Lago following Trump's statements about white supremacist groups protesting in Charlottesville, Virginia earlier this month.