Complex sentence: Woodhull soon recruited his brother-in-law Amos Underhill to gather intelligence, who ran a boarding house in the city with his wife Mary, but Underhill's reports were often too vague to be of much value.
Split into simpler sentences: Woodhull soon recruited his brother-in-law, Amos Underhill, who ran a boarding house in the city with his wife Mary, to gather intelligence. But Underhill's reports were often too vague to be of much value.