I was sitting in my office when the phone rang with the caller ID of “Trump Org.” “Oh no,” I thought. “It can’t be.” Ever since my book Billionaires: Reflections on the Upper Crust came out in September, weird things had happened. But hearing from Donald Trump’s organization was among the most bizarre.

In my book, I had recounted an amusing interaction with the New York billionaire. In 2012, I had been asked for a press comment about Trump speaking at the GOP convention. Tongue in cheek, I had replied that Republicans should send him on an all-expenses-paid trip around the world because if he spoke at the convention, he would bring the party nothing but trouble. The morning my quote was published, I got a phone call from Trump’s assistant. She asked for my email address, and after I gave it to her, she sent me a missive from the billionaire himself. He had pasted my comment about him into the body of the email and written in big, black, bold letters, “Darrell, You are a ‘fool’. Best wishes, Donald J. Trump.”

When the book came out, it generated no immediate response from the New York billionaire. But after I published a Political Power Index at that ranked Trump No. 23 on the list of wealthy individuals who had influenced the 2014 elections, his top executive called, complaining that his boss deserved a much better rating. How was the list devised, he asked? What criteria went into its compilation? As I explained that the rating was based on campaign contributions, issue advocacy, grassroots funding, and endorsements, among other things, this individual argued that my ranking of his CEO was way too low.

Other than the Koch brothers, who had spent hundreds of millions of dollars to elect a Republican Congress, who had more influence in 2014, he asked. According to him, Trump was the key person in this year’s GOP triumph, much more so than nearly all of the other people on the list. Trump had done endorsements and appearances, and recorded calls on behalf of a number of Republican candidates. I asked him how he thought Trump had swung the election. He pointed to robocalls and claimed Trump had turned around several close races. Indeed, the executive noted that according to their information, call recipients had listened to Trump’s robocall messages in their entirety as opposed to the average length of 8 to 10 seconds for other callers. That was impact, this man claimed, and evidence why Trump had been short-changed in my power index.

This unhappy response was not unusual. When I wrote Billionaires, I did not anticipate the outpouring of emotion it would generate from the super-wealthy themselves, despite the fact that I had seen the difficulties other writers had encountered when tackling the contentious topic of wealth in America. For example, Jane Mayer of The New Yorker wrote a critical article about conservative billionaires Charles and David Koch and became the object of a smear campaign of unknown origins that questioned her professionalism and personal integrity.

Following the publication of his best-selling book, Capital in the 21st Century, economist Thomas Piketty was accused of being leftist for noting that the value of capital was rising faster than wages and therefore exacerbating inequality. It didn’t matter that multi-billionaire Bill Gates agreed with Piketty, writing that inequality was a problem and governments could play an important role in offsetting the negative ramifications of inequity.

Others have detailed the legal and personal risks of criticizing the super-wealthy. In his book about the Koch family, Sons of Wichita, author Daniel Schulman documents family members’ uses of private detectives, legal threats, and press campaigns to deal with adversaries.

Based on these and other examples, it is clear that many billionaires are sensitive about their public portrayals—and have the resources to act on their discontent. Billionaires are a confident lot, I concluded. When they see things they don’t like, they go right to the source and demand action. These individuals didn’t make a billion dollars by being meek or timid.