what is the main point of this book?

You are really asking a thematic question here. There are actually many themes. I think one of the major themes is respect and reverence for the land. The earth also quite literally evokes tradition and ancestry. By the end of the novel those characters closest to the earth -- Wang Lung's father, O-lan, and Ching -- have been consigned to the earth. They are the earth once more, just like their ancestors. Their lessons, their wisdom, and even their bodies thus enter the cycle of feast and famine that follows from the earth.Another one is Cyclical time.

When the characters of the newer generations become caught up in "new" things -- the revolution, city fashions, etc. -- at the expense of "old" things -- the gods, the ways of farming -- beware. The new things aren't really new, and concentrating too intently on them blinds characters to the lessons of the past. Those who see the cycles of the earth and its people, and link their lives up with these cycles, thrive; those who don't, perish.