President Trump on Sunday suggested late-summer reading for his 37 million Twitter followers in a tweet that could get anyone else in his office fired.

The book, “Cop Under Fire,” was written by Milwaukee County Sheriff and Trump campaign supporter David Clarke, Jr., who had pinned a link to the book’s Amazon page on his Twitter profile in February.

Trump shared the Amazon link during a weekend of Hurricane Harvey-related tweets.

Under federal ethics regulations, executive branch employees are prohibited from using their position “to endorse any product, service or enterprise…” The president and vice president are exempt, however.

While technically legal, the tweet was criticized by government ethics groups as the latest in a string of rule-bending behavior by the Trump administration.

In February, the House Committee on Oversight and Government Reform wrote a letter to the Office of Government Ethics (OGE) accusing Kellyanne Conway of violating federal ethics laws after she promoted Ivanka Trump’s fashion line on Fox News. OGE advised action against Conway, but the request was rebuffed by the Office of the White House Counsel.

“This is only the latest unfortunate example of President Trump disregarding the ethical norms that help ensure that public officials are working for the public rather than for private interests,” Brendan Fischer, an attorney with the Campaign Legal Center, wrote in an email.

“And when coupled with some of the president’s other recent acts — the pardon of Joe Arpaio, for example — there is a sense that one of the administration’s top priorities is to use the power of government to reward those individuals and special interests who’ve ingratiated themselves with the president.”

During the 2016 campaign, Clarke was part of a cross-country bus tour campaigning for Trump. The tour was sponsored by the Great America PAC, the leading PAC supporting Trump on the campaign. Clarke was also briefly in the running for a job as assistant secretary in the Department of Homeland Security.

Clarke’s Twitter profile features multiple photos of Trump flashing a thumb’s up.

Citizens for Responsibility and Ethics criticized Trump’s endorsement as well, accusing the president of using his official account “for an advertisement to benefit a major campaign supporter.”

Clarke’s book isn’t the only one on Trump’s recommended list. He’s also promoted two by conservative author Nick Adams.