Short answers, pulled from the story.
Allen Klein was an American businessman born on the 18th of December 1931 in Newark, New Jersey, who became famous for simultaneously managing the Beatles and the Rolling Stones. He founded ABKCO Music and Records and pioneered aggressive contract renegotiations that changed how recording artists were compensated, though his methods also led to decades of litigation with his clients.
Klein negotiated a deal with Decca Records that initially secured the Rolling Stones $600,000, then renegotiated to guarantee them $2.6 million, more than the Beatles were earning at the time. He also raised the band's royalty rate and eliminated a 25% management commission. However, the Stones later sued Klein for withholding royalties and controlling their publishing catalog through ABKCO for decades.
Three of the four Beatles chose Klein after John Lennon publicly warned that the band would be broke within six months. Klein met Lennon on the 26th of January 1969 and was retained that same day. George Harrison and Ringo Starr sided with Klein over Paul McCartney's preferred representatives, and Klein was given a three-year management contract on the 8th of May 1969.
Klein was convicted of a single misdemeanor charge for making a false statement on his 1972 tax return. After his first trial ended in a mistrial, a 1979 retrial found him not guilty of three felony tax evasion charges but guilty of the misdemeanor. He was fined $5,000 and served two months in jail from July through September 1980.
Klein organized the financial and artist logistics for George Harrison's Concert for Bangladesh at Madison Square Garden in 1971, convincing Capitol Records to grant a 50% royalty rate and persuading artists including Bob Dylan and Eric Clapton to donate their shares to charity. The concert raised over $15 million, but Klein failed to register the shows as a UNICEF charity event, which denied the proceeds tax-exempt status and led to the IRS holding back $10 million for years.
Klein set up companies he owned as intermediaries between his clients and their record labels. These companies owned the rights to the music, financed recording, and sold the results to labels, then paid royalties and advances to the artists over extended periods. Klein invested the pooled money during those periods and kept the returns above what he was obligated to pay, enriching himself alongside his clients.