One idea to keep in mind when you're reading Spoon River is that it is a Modern American work; so you will often recognize the theme or elements of disillusionment with life, especially with American "simple" life. Here are the themes:
1. "Mrs. Sibley"--she goes to her grave disillusioned with all that should be magical in life (stars, new growth, and relationships with the opposite sex). Her last line,
"My secret: Under a mound that you shall never find."
implies that she feels she has gotten the last laugh, most likely at her expense.
2. "Ernest Hyde"--His epitaph's theme is the change in a man's view of the world as he ages. In youth, he believes that all that he sees is truly the way things are ("My mind was a mirror"--line 1), but as one ages, reality sets in and "scratches" or mars one's soul. Masters writes that "a mirror scratched reflects no image."
3. "Mabel Osborne"--She represents the longing of a soul to fit in and be adopted by its society. She compares herself to the beautiful, thirsty geranium planted on her grave. Throughout her life, she longed or thirsted for acceptance from her town; the townspeople recognized her longing yet did nothing just as they recognize the geranium's need but allow it to wilt. Mabel is the person who has desires but is too timid to voice those desires or ambitions.
4. "Doctor Meyers"--Again, the doctor's "last words" demonstrate disillusionment but of a kind different from Mrs. Sibley's or Ernest Hyde's. He actually seemed to live a charmed life with a happy marriage, good children, and a fulfilling career. Masters uses his character to show a version of fatalism--no matter how well someone lives, he does not control his ending. Thus, the doctor tries to help someone, loses his reputation, his wife, and finally his life.
5. "Frank Drummer"--He seems to be the village idiot to the people of Spoon River because he cannot verbally express all that his mind holds. He dies at a young age, but ironically saw life as a "cell" and death as a "darkened space"--not much worse than his life. For the postmodern reader, the theme from "Frank Drummer" is that people are not always what they appear to be and that often those who appear to be "simple" have the "clearest vision" of how life truly is.
Again, as you read Masters' poems, remember that each person is speaking from the grave and presenting a lesson about life. That lesson is almost always negative because of the literary era to which Masters belonged. I hope this helps!