The idiom "to plumb the depths" means to reach the deepest point. So... in your example, the sentence refers to her novels regularly reaching the deepest point or aspect of human despair. The word "plumb" in this idiom comes from the use of a "plumb line" which was a long cord with a heavy metal weight attached to one end. A plumb line was used to measure the depth of water under a ship; in other words, the deepest point.

I come from China, hand over a friend