I didn't remember many details about Senselessness beyond that it was about a guy who gets a job copyediting 1100 pages of firsthand testimonies by indigenous people and activists who survived the brutal massacres of countless people at the hands of the military in an unnamed country. Our narrator is not always sympathetic, but he slips into a state of madness from reading -- absorbing -- these horrors day in and out. "I am not complete in the mind," is the opening passage of the book, a quote from one of these indigenous survivors, but a reflection as well of the narrator's own state. The reader must wonder if his growing paranoia is justified, or if it's only that he is not complete in the mind.
On this reading, I read Senselessness as a sort of sequel to Castellanos Moya's book, The Dream of My Return, which I've also read twice, but it seems to me in the wrong order. I should go back and read it now with this fresh in my mind. In The Dream of My Return, the narrator is on the verge of leaving Mexico City for his home country, where his life may be in danger, but where he feels a compulsion to return, risking his own life. This potentially dangerous copyediting job could be the thing that awaits him there.