、。
Her heart pounding, she read the letter.
Literal
She [topic-は] chest [object-を] [pounding-どきどき] caused-to-be, that letter [object-を] read.
どきどき is one of Japan's most recognizable mimetics — for the rapid, pounding heartbeat of nervousness, excitement, fear, or romantic anticipation. The frame ~をどきどきさせる (literally 'cause to be どきどき') turns the mimetic into a verbal complement via the causative させる. The te-form chains the heart-pounding into the action of reading: she set her heart pounding, and read the letter. A vivid, cinematic line — the kind that opens a romance novel chapter.