。
She treasured his old love letters.
Literal
She [topic-は] his [genitive-の] old [genitive-の] love-letter [object-を] treasure [into-に] doing was.
大切にする ('to treasure, value, take good care of') is a foundational set phrase, applied to people, possessions, memories, time. The ~ていた form here marks the long-term ongoing state of treasuring — over time, not just a moment. 恋文 (こいぶみ, 'love letter') is a Sino-Japanese-flavored word — somewhat literary, slightly old-fashioned. Modern equivalents include ラブレター (loanword) and 恋人への手紙 (descriptive). 昔の literally 'of long ago' is functioning attributively: 'old love letters.'