。
She cried while reading the letter.
Literal
She [topic-は] letter [object-を] reading [while-ながら] cried.
~ながら marks two simultaneous actions: reading and crying happened at the same time. The main action (crying) comes at the end, while the secondary action (reading) precedes ながら. This is the simultaneous-action ながら, distinct from the concessive ながら (despite). The emotional weight is in the juxtaposition — whatever the letter contained moved her to tears as she read.