WASHINGTON—Shortly before his death in February 2016, Supreme Court Justice Antonin Scalia spoke favorably of Donald Trump’s presidential run.

“Justice Scalia thought it was most refreshing to have a candidate who was pretty much unfiltered and utterly frank,” said the late jurist’s literary collaborator, Bryan Garner, a legal dictionary editor who spent two weeks in 2016 traveling with Justice Scalia through several Asian countries.

The justice thought well of Scott Walker, the Wisconsin governor whose campaign for the Republican nomination stalled, said Mr. Garner, whose memoir of a decadelong friendship, “Nino and Me,” comes out Tuesday. “But he was fascinated by the fact that Trump was so outspoken in an unfiltered way, and therefore we were seeing something a little more genuine than a candidate whose every utterance is airbrushed,” Mr. Garner said in an interview.

While Justice Scalia may have approved of many of Mr. Trump’s conservative judicial nominations, Mr. Garner declined to speculate on how he might have viewed other aspects of his presidency. “These [were] early days in the campaign. It shouldn’t be looked at through the lens of everything that’s happened since,” he said.