Why does the object come first in the sentence 这本书我要再看一遍?
Many languages have this type of sentence construction to put topic first. It is not unique in Chinese.

You can find it in English too. "This dress, I really want to buy". This dress is the topic of sentence. In Chinese 這本書，我要再看一遍。這本書 is the topic of the sentence.

This kind of construction is quite common in classic Chinese. In Mencius, "魚，我所欲也。熊掌，亦我所欲也。" 魚 and 熊掌 is the topic of a sentence.

In Japanese, putting topic with "は" construction is even more common.