Barnes & Noble has been acquired by the hedge fund Elliott Advisors for $638 million, a move that has momentarily calmed fears among publishers and agents that the largest bookstore chain in the United States might collapse after one of the most tumultuous periods in its history. The sale was announced Friday morning after months of speculation over the future of Barnes & Noble. The acquisition follows Elliott’s purchase of the British bookstore chain Waterstones in June 2018. James Daunt, the chief executive of Waterstones, will also act as Barnes & Noble’s C.E.O. and will be based in New York.