请问all of which和all of them的区别

请问all of which和all of them的区别
之前做题目看到如下例句：
He planned to write 3 books about math,but ______ never published.选项中有all of them 跟all of which 可以选,结果答案是all of which.敝人实在难以分辨其差别,有没有哪位仁兄可教我?
另外还有一例句如下：
His notebooks were filled with fantastic inventions and machines,but ______ were built.
同样也有few of them和few of which 可以选,这次答案却是few of them...