。
She read the letter and seemed sad.
Literal
She [topic-は] that letter [object-を] read [and-て] sad-looking was.
~そうだった is the past form of the visual evidential — describing an inferred past impression based on appearance. The ~て-form of 読む links the action of reading to the state of looking sad, with implied causation: she read it and as a result appeared sad. 悲しそう uses ~そう attached to the い-adjective stem 悲し — strictly an outside observation, not a claim about her actual inner state. To assert her felt sadness directly, one would say 彼女は悲しかった.