Added to the shyness of his look was a faint streak of melancholy.

【翻译】他的神情除了羞涩之外，还有一丝淡淡的忧郁。

这是一种特殊的倒装句。结构为：过去分词短语 + be + 主语。

正常语序为：

A faint streak of melancholy was added to the shyness of his look.

在他那羞涩的神色上，又添了一抹淡淡的忧郁。

语义效果不同。

又如：

Seated beside the teacher were some small children.

Cut in the ring was a name: Agnes.

Scattered on the floor were several books and magazines.

Ranked second and third were grocery and electronics customers.