Complex sentence: In 1930 he published his first book "The Seeds of Murder" which introduced Captain Hugh North, a detective in Army Intelligence and the hero in a long series of "intrigue" novels.
Split into simpler sentences: In 1930 he published his first novel, "The Seeds of Murder", which introduced Captain Hugh North, an agent of U.S. Army Intelligence. North was the hero in a long series of "intrigue" novels.