-What do you think of the book -Oh,excellent.It's worth _____a second time.1、横线上为什么要填reading 而不填being read?2、second 前面为什么要用a?

1.词组be worth doing。
2.序数词前用不定冠词a而不用定冠词the，表示“又一，再一”的意思。
e.g.Please try a second time.请再试一次。

be worth doing sth. "……某事值得被做"
The question is not worth discussing again and again.
这个句型本身具有被动，所以reading.
second 前加a是在一 又一的意思

be worth doing,be worth to do,be worthy of (doing)“值得”只有这几种用法
a second time是固定短语,表示“再一次”或“又一次”,"a"是不定贯词,不表特指.

worth doing,无被动
a second time 固定用法