In contrast, according to Mr. Thae’s book, Kim Jong-chol receives full protection from his younger brother, enjoying perks available for family members. Little is known about Kim Jong-chol except that he and Kim Jong-un studied in Switzerland as teenagers and that he was an avid guitarist.

In a series of encrypted emails in 2015, Mr. Thae — who at the time was the No. 2 diplomat in North Korea’s London Embassy — received instructions for handling a special visitor from Pyongyang: Kim Jong-chol. He wanted to attend an Eric Clapton concert there.

The leader’s brother was such a fan of Mr. Clapton that the North Korean government once made a deposit of one million euros, about $1,170,000, on a contract to invite him to perform in Pyongyang, Mr. Thae said. But the musician declined and the deposit was returned.

Image An undated photograph of Kim Jong-un’s mother, Ko Young-hee.

For the London concert, the book says, Mr. Thae was ordered to secure six tickets and two suites at one of the city’s best hotels and to make reservations at top restaurants. After the brother landed in London late at night, flying first-class via Moscow, he ordered Mr. Thae to take him straight to the HMV music store on Oxford Street and to bang on the door to have the store opened for him.

“On my way to London, all I could think of in the plane was visiting the music shop,” Mr. Thae quotes him as saying.

Mr. Kim’s brother was eventually persuaded to instead visit the shop when it opened the next day. But Mr. Thae said the trouble for him and his staff was just beginning. The brother insisted on smoking wherever and whenever he wanted. And despite all the scouting for top-notch restaurants, he also liked to eat at McDonald’s.