Complex sentence: On May 15,1832, Halleck became the private secretary to John Jacob Astor, who also appointed him as one of the original trustees of the Astor Library of New York, later the Public Library.
Split into simpler sentences: On May 15,1832, Halleck became the private secretary to John Jacob Astor. The wealthy fur trader merchant turned philanthropist later appointed him as one of the original trustees of the Astor Library of New York, later the Public Library.