What is the theme in Humboldt's Gift by Saul Bellow?

The nature of death is a theme to consider. Charlie is middle-aged man obsessively aware of his own mortality. Perhaps forty percent of the narrative in Humboldt's Gift happens inside of Charlie's head in the form of memories and lengthy flashbacks. He clearly states his belief in the existence of the human spirit and soul after the rite of passage called death. Further, he speculates that we the living have an obligation to carry out the work of the dead and, on more than one occasion, sarcastically quips that we have a right to profit from them. As Bellow's novel winds down, the reader finds Charlie locking himself in his humble room in a boarding house in Madrid, spending hours reading to and talking with the dead. The novel ends with Charlie favoring the idea of retreating to Switzerland to spend time at a Mecca for practitioners of anthroposophy, the esoteric framework for his beliefs on the nature of death.