Since the release of the first novel, Harry Potter and the Philosopher's Stone, on 30 June 1997, the books have gained immense popularity, critical acclaim and commercial success worldwide. They attracted a wide adult audience, and have remained one of the preeminent cornerstones of young adult literature.[2] The series has also had some share of criticism, including concern about the increasingly dark tone as the series progressed, as well as the often gruesome and graphic violence depicted in the series. As of May 2015, the books have sold more than 450 million copies worldwide, making the series the best-selling book series in history, and have been translated into 73 languages.[3][4] The last four books consecutively set records as the fastest-selling books in history, with the final installment selling roughly 11 million copies in the United States within the first 24 hours of its release.