Complex sentence: Smith became one of the preeminent religious studies scholars in the United States, his book "The World's Re ligions" being a classic in the field, having sold over two million copies and being considered a particularly useful introduction to comparative religion.
Split into simpler sentences: Smith became one of the preeminent religious studies scholars in the United States. His book "The World's Religions" is a classic in the field, sold over two million copies and is considered a particularly useful introduction to comparative religion.