When the paperback version of Senate Majority Leader Mitch McConnell’s 2016 memoir “The Long Game” came out in December, McConnell got some promotional help from an important social media influencer. President Trump, who wrote a new foreword to the book, tweeted praise for it twice to his more than 80 million followers.

“A great new book,” Trump called it in one tweet. He added: “Amazing story!”

Trump’s glowing endorsement came just in time for Christmas, but perhaps he’d been meaning to show McConnell (R-Ky.) just how much he cared: The following month, the Senate began hearing arguments in Trump’s impeachment trial, before ultimately voting to acquit him.

Trump’s well-timed plug for McConnell’s book is part of a recurring, if largely overlooked, feature of his presidency. Since taking office in early 2017, he has tweeted or retweeted 141 times on behalf of authors and their new books, making the topic one of his leading Twitter preoccupations. He typically offers the same blurb-worthy comment: “Great new book!”

Like his McConnell tweets, Trump’s endorsements are striking because of how politically strategic they’ve been. Collectively, they may help reinforce loyalty to him. They may also be a kind of in-kind contribution, given that a presidential pat on the back could help sell a book and thus increase an author’s royalties.

Many of the books he’s promoted have been those by political allies in Congress. The list includes Republicans such as Sens. Tom Cotton (Ark.), Tim Scott (S.C.), Joni Ernst (Iowa), Martha McSally (Ariz.), Mike Lee (Utah) and Rand Paul (Ky.), and Reps. Dan Crenshaw (Tex.) and Steve Scalise (La.). Trump has never endorsed a book by an elected Democrat or a local official of any party while in office.

A second major focus of Trump’s tweets are books by conservative media figures, particularly those employed by Fox News. The list here includes Fox hosts and pundits Tucker Carlson, Jeanine Pirro, Mark Levin, Brian Kilmeade, Ainsley Earhardt, Pete Hegseth, Dan Bongino, Gregg Jarrett, Rachel Campos-Duffy and Andrew McCarthy. All of these authors have been reliable supporters of the president, frequently defending him on the air.

Trump has made no mention of recent books by Fox News anchors Bret Baier and Chris Wallace, both of whom have at times reported unflattering news about the administration (Fox News declined to comment).

A third category for Trump are books that take his side of an argument or otherwise flatter him. Trump repeatedly tweeted about Jarrett’s “The Russia Hoax: The Illicit Scheme to Clear Hillary Clinton and Frame Donald Trump” and aboutThe MAGA Doctrine,” a book by Charlie Kirk, who heads a pro-Trump nonprofit. A book by Fox News pundit Nick Adams that compares Trump to Winston Churchill has also received presidential praise on Twitter.

The sheer number of Trump’s book endorsements is unprecedented for a sitting president. President Barack Obama made book recommendations while in office, too, but his list was substantially different from Trump’s. Obama tended to recommend historical works (Ron Chernow’s biography of George Washington), literary classics (Gabriel García Márquez’s “One Hundred Years of Solitude” and F. Scott Fitzgerald’s “The Great Gatsby”) and recent fiction (Anthony Doerr’s “All the Light We Cannot See”). None of his choices were titles about himself or his presidency.

Federal rules prohibit government employees from promoting products of any kind, including books, but the Office of Government Ethics specifically exempts the president and vice president.

Nevertheless, the practice is “enough to make people question” what Trump is doing, said Jordan Libowitz, a spokesman for the government watchdog group Citizens for Responsibility and Ethics in Washington.

“There seems to be a symbiotic relationship” between Trump and the authors who laud him, he said. “They praise him, he endorses [their books]. It’s almost like a little ecosystem. If you give him enough praise, he might be able to make you some money” via an endorsement.

Added Libowitz: “People imagine the president to be above this kind of thing. But Trump has shown from the beginning that he doesn’t tend to follow ethical norms.”

It’s not clear how many of the books that Trump has endorsed he’s actually read. He rarely mentions books in any public forum other than Twitter. Before running for president, he almost never cited books other than his own. (The White House press office declined several requests for comment.)

It’s also not clear how, or whether, a presidential endorsement affects a book’s sales. Several Trump-backed volumes have indeed become bestsellers, though a number of these benefited from large bulk purchases from Republican organizations that gave the books away to political donors (among the authors praised by Trump who reached bestseller status this way: Donald Trump Jr. and Nikki Haley, Trump’s former ambassador to the United Nations).

Some of the books that Trump has endorsed, such as those by Pirro and Levin, were bestsellers by the time the president got around to mentioning them. On the other hand, “endorsements by prominent people often spur book sales,” said Bradley Graham, the co-owner of Politics and Prose, the Washington bookstore. “And no one is more prominent than the president.”

Even so, the volume of Trump’s book endorsements may be counterproductive, said Thomas Dunne, who spent nearly 50 years in the publishing business, handling books by the likes of Newt Gingrich, Pat Buchanan and Bernie Sanders.

“In general, I think he now sends out so many tweets, the impact of just one plug would be minimal,” he said.

What seems more apparent, said Dunne, is the inverse: Trump’s attacks on various books have hyped their sales. He said, for example, that “Team of Vipers,” a tell-all by a former Trump administration official named Cliff Sims, got a bump after Trump blasted it on Twitter (a “boring book based on made up stories and fiction”). The president’s denunciation fueled more mainstream media attention, which may have triggered more sales, Dunne said.

The same kind of negative Twitter attention from Trump may have helped sell other critical works about him, including bestsellers by Michael Wolff, Bob Woodward, former FBI director James B. Comey and former Trump national security adviser John Bolton.

So far, Trump hasn’t mentioned a forthcoming book by his niece, Mary L. Trump (“Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man”), which has become a bestseller based on preorders.