Hello Sandy! How are you?
This answer is based in Perfect Participle's explanation. Well, We use the perfect participle to emphasize that the first action was completed before the second action started. So the action of "reading" was before the action of "was looking forward..."
Sandy, the problem is that you are a little confuse about present participle and perfect participle... don't get sad, this two verb tense makes us confusing kkkk it's normal be confuse with this verb tense, when I was searching to answer you I confess that I stayed a little confuse too ahahah
anyway, I found this answer in chinese, I don't know to read Chinese, but I hope that it can help you ahaha xD
having done作状语时 表主动 且动作发生在主语前
doing作状语时 表主动 通常表示动作和主句同时发生
这句话的意思是 在去年假期的时候读完这本书后 我很期待看看这部电影
不过好像这句话的主句有点问题 应该是look forward to seeing吧..
Aaah I found another site that can be helpful too:
Sandy, I hope I've helped you a little bit ahaha ;D