。
She's writing a letter right now.
Literal
She [topic-は] now letter [object-を] is-writing [right-in-the-middle-ところ] is.
~ているところ tightens the temporal frame of ~ている from 'is in some ongoing state' to 'is precisely at this point in the activity.' The English 'right now' or 'in the middle of' captures the emphasis. Letter-writing remains a cultural touchpoint in Japan: handwritten 年賀状 (New Year's cards) and 暑中見舞い (summer greetings) are still posted by millions, and personal letters carry weight as a more thoughtful gesture than email or messaging.