"He cheats at the highest level. He cheats when people are watching, and he cheats when they aren’t," he writes in Commander in Cheat: How Golf Explains Trump, adding that he came to the conclusion after interviewing many people who had played with the president.
"Whether you’re his pharmacist of Tiger Woods, if you’re playing golf with him, he’s going to cheat,” he says.
Both Secret Service agents and the president's caddies routinely move his balls out of difficult positions, which helps him record lower scores, he adds.
Mr Trump has repeatedly denied that he cheats at golf.
The website TrumpGolfCount has suggest that the US leader has made 170 Trump golf club visits during his time in the Oval Office, compared to Mr Obama's 66 visits in the same number of weeks into his presidency.
A review of US Federal Government spending data by the liberal political action committee (PAC) American Bridge 21st Century, suggested that the Secret Service had spent nearly $1m (£777,000) in expenses to protect the president on his golf trips.
A PAC is an organisation that pools campaign contributions from members and donates those funds to campaigns for or against candidates.
American Bridge 21st Century said the spending data showed that $769,520 had been spent on 65 presidential golf outings. A further $200,000 had been spent by the Secret Service to protect the president at Mar-A-Lago, Mr Trump's private club in Florida where he regularly hits the greens.
The figures cover contracts for equipment, such as privacy fencing, portable bathrooms, tents, and barricades.
While presidents have enjoyed a round of golf, Mr Trump has often played at his own courses which are more difficult to protect.
Barack Obama was also a keen golfer but often played on courses located on military bases.
Mr Trump has claimed he has a 2.8 handicap and has won 18 club championships. Mr Reilly described this as “a lie that’s so over-the-top Crazytown, it loses all credibility among golfers the second it’s out of his mouth.”
Jack Nicklaus, widely considered to be the greatest golfer of all time, has a handicap of 3.7. He is just seven-years older than Mr Trump.
The author has routinely accused the president of cheating and in 2016 Mr Trump responded by saying: “I always thought he was a terrible writer. I absolutely killed him, and he wrote very inaccurately.”
In 2012, when Alice Cooper was asked who was the worst celebrity golf cheat, he replied: “I wish I could tell you that. It would be a shocker. I played with Donald Trump one time. That’s all I’m going to say.”