"La muerte es una joda" is the letter written to an old friend by a dying man. He is Argentine or Uruguayan, who moved to Mexico City after receiving a cancer diagnosis. Although he admits he is frightened of death, the letter is written with something of a light, almost joking, tone. He is already getting fainting spells, and he suspects the next time he faints, he will not be awakening.
"Un sabor ácido" is a letter to a childhood friend. After some brief thoughts on solitude and then some reminiscence regarding his childhood, the author goes on to explain why he has written the letter. He is in prison for shooting his wife. He had developed a problem with jealousy, and had hired a detective to follow her. The detective he hired worked alone, not with an agency, which he figured was a point in his favor. The detective had reported that she was having an affair. Enraged, the man shot his wife. He then flees and hides out in the house of another friend. Several days later, he finds out that his wife was not cheating on him, and that the detective worked alone because he was disreputable. He had a reputation for fabricating tales of infidelity concerning the wives he was watching.
There's a sense I get in this collection of these being very much the stories of a writer in his sunset. A lot of sort of bittersweet coming to terms with loss. This is pretty strong in the first story, but also comes through in the second story's nostalgic reminiscence. The theme of solitude seems to be most ironically used in "sabor," where it appears the narrators preference for solitude ends up dooming him.