A new book out this month reveals previously-unknown details about the 2016 Trump campaign, and how one of its top officials became a key witness in the probe of Russian election interference.

Daphne Barak, author of "To Plea or Not to Plea: The Story of Rick Gates and the Mueller Investigation" (published by Center Street), spoke exclusively to Gates, who's been convicted of financial fraud and making false statements to the FBI.

Gates continues to cooperate with the government – he is scheduled to testify in the Justice Department's upcoming trial against another longtime Trump associate, Roger Stone – as he awaits sentencing.

In the book, Gates talks about how he went from being a close confidant of President Trump to a convicted felon.

Speaking publicly for the first time since he was charged by special counsel Robert Mueller in October 2017, Gates told Barak (who says she is a close friend of the president's), "The Trump campaign, it was not a very sophisticated operation, but it was a very effective and smart operation."

Gates was by Mr. Trump's side from the very early days of the campaign, but he was frozen out after he came under scrutiny in the special counsel's investigation for his lobbying work in Ukraine.

"His life was interrupted; at one moment everything stopped," Barak told correspondent Paula Reid.

Barak spoke to Gates about why he finally decided to make a deal with Mueller and flip on his former boss and Trump campaign chairman Paul Manafort. "He had to plea because it's millions [sic]," she said, "and the government would just come with more and more charges."

Gates was indicted by a federal grand jury investigating Russia's interference in the 2016 election, on eight counts including fraud and money laundering. The indictment noted that between at least 2006 and 2015, Manafort and Gates worked as unregistered agents lobbying for the government of Ukraine, and took in tens of millions of dollars in income. The pair allegedly laundered the money through "scores" of U.S. and foreign corporations and bank accounts, including banks in Cyprus, Saint Vincent, the Grenadines and the Seychelles. Gates ultimately pleaded guilty to two counts, and agreed to cooperate with the government, testifying against Manafort in Summer 2018.

During his blockbuster testimony, lawyers pressed Gates on his past infidelity as well as stealing from his former boss.

"Rick Gates is a convicted liar; why should we believe anything he told you?" Reid asked Barak.

"It would always be a stigma that all these people are convicted liars," said Barak. "On the other hand, in the Mueller report, they found his testimony very credible."

Gates also opened up to Barak about behind-the-scenes details from the campaign, including whom Donald Trump may have wanted as a running mate. He said Indiana Governor Mike Pence was not then-candidate Trump's first pick.

"To be, you know, candid, he wasn't at the top of the list at that moment," Gates told Barak. "And rightfully so. We had just started. And we had others to vet."

He revealed that it was Melania Trump who insisted Pence be her husband's running mate because she didn't think he would run against Mr. Trump in 2020.

Despite no longer being in the Trump inner circle, in this book Gates speaks glowingly about the entire Trump family. But Barak told Reid the president has not spoken with Gates about this book or about his decision to plead guilty and cooperate with the prosecution of Mr. Trump's former colleagues.