Davy Sidjanski, who headed the international children's book publisher Nord-Sued Verlag, died November 8 of heart failure. He was 49. Sidjanski had led Nord-Sued since 1979 when he acquired the publisher from his parents. Under his leadership, Nord-Sued expanded the practice of coproducing books simultaneously in different languages. The U.S. division of the company, North-South Books, was launched in 1984. Sidjanski sold the entire publishing operation to a group of investors this spring.