Although he developed the phrase "fifteen minutes of fame," artist Andy Warhol's time in the spotlight was much longer. Throughout his long and distinguished career, Warhol created many paintings and pieces of artwork that are considered to be masterpieces. His impressive collection of works ranges from portraits of acclaimed American icons Grace Kelly and Marilyn Monroe, to profiles of the more controversial figures, including Mao Zedong. Although he is remembered primarily as an artist, Andy Warhol authored several books, including The Philosophy of Andy Warhol, and Popism: The Warhol Sixties; he also founded Interview Magazine before his death in 1987. Andy Warhol is known for his unique and innovative style of art that examines celebrity culture, fame, and the world of advertising and branding, with traditional artistic techniques and mediums. This revolutionary style developed the title of "pop art," and found a permanent home in the world of the arts. Many of Andy Warhol's best works were produced in his personal art studio, which he referred to as "The Factory." Although he called that little space his own, Warhol shared it with many others. The Factory quickly became a popular hangout spot for his circle of distinguished peers and celebrity friends; it was also frequented by Bohemian street characters, fellow artists, and wealthy clients, all of whom were inspired and entertained by the charismatic Warhol. Many also found themselves the subjects of his artistic expressions, which were translated through hand drawings, prints, photographs, silk screens, sculptures, and films. Although he is now deceased, Andy Warhol's spirit is alive and well in the artwork that he left behind. His pictures, paintings, and sculptures are among the most valuable in the world, with prices ranging from hundreds of thousands of dollars, to $100 million. These works are considered by many art collectors to be cherished treasures.