-what do you think of the book?

-Ok,excellent. It's worth reading a second time.

It是指the book,还是指 reading a second time这个动作? 

还向老师请教一下,这种情况下,句子里think of可以用think about代替吗?

     【答】原句中的It是指the book，即： It's worth reading a second time. = The book is worth reading a second time. = It is worth reading the book a second time.

（在最后的句子中，It是形式主语， worth 是表语，reading the book a second time 是真正主语）

       这种情况下,句子里think of 不可以用think about代替。