Martha's letters represent some of the thematic concerns of "The Things They Carried" that are expressed through the character of Jimmy Cross. Martha and Jimmy set up a dichotomous contrast between life without war and life with war. This is one reason that O'Brien's story resonates with so many and for so long: universal themes of living are the focal point of the story, which therefore is far more than a story about soldiers in war.
The symbolic weight of Martha's letters in one regard is that they symbolize the theme of life versus war. Martha's letters are filled with the images of life, such as "walking barefoot along the Jersey shore" where bits of living reality are collected. Martha's bits of life, like the pebble, bring life and war together, like the pebble brought the sea and shore together creating a symbolic unity out of actual disparity:
Martha ... found the pebble on the Jersey shoreline, precisely where the land touched water at high tide, where things came together but also separated. ...[in] this separate-but-together quality,
Jimmy's feelings and thoughts (aside from inevitable love and longing) are best expressed in the narrator's comment:
Martha, carrying nothing. [Jimmy] would feel himself rising. Sun and waves and gentle winds, all love and lightness.
According to this description, Jimmy might be said to feel the distance between the separate worlds; he feels the distracted movement of his soul away from the weight of his present duty. Jimmy's thoughts (aside from the obvious thoughts of Martha) are for the duty he owes to his soldiers: to keep them safe and alive despite the burden of war they all carry and despite the burden of responsibility for life that Jimmy carries.
Two other elements of the symbolic weight of Martha's letters are embedded in the other themes of lightness versus weightedness and love versus death. Jimmy's thoughts and feelings are as divided on these themes as the sea is divided from the shore. Jimmy lives in weightedness and longs for lightness; Jimmy dreams of love and faces death. Jimmy's other thoughts are similarly revealed in his reactions to Martha's letters.
In order to delve further into Jimmy's feelings and thoughts related to these other two themes, analyze Jimmy's behavior and reported words in instances where weight and death enter Jimmy's story.