at that time指某一个回忆图像的时间点,这个点在过去的某一刻,此处应翻译为“当时”,根据图像,他当时“正在”忙,用he was working更具体,如果用he worked in the library,这句话的意思是他原来在图书馆工作,（即现在在哪工作就不一定了）和他“当时”是否在工作就没什么关系了.
所以有这几种情况“
他当时正在工作he was working in the library
他原来在那工作he worked in the library或he used to work in...
他已经在那工作好几年了he has been working in the library for several years.
他现在仍在这里工作he's still working in the library
他的职业是...he works in a library
出那件事之前他在那里工作（即现在不在了）he had been working in the library before it happened.
如果他不是因为那件事就会在图书馆工作了（即因为那件事所以没在图书馆工作成）he would have worked in the library if it wasn't for that thing.