which of books are interesting??
主要区别于定语从句的关联词。
which ,which是疑问代词，而them 是代词，二者不在一起挨着用，而which of 是代词加介词，of 在这里是指范围，即这些书哪本有趣呢？
所以___ them books are interesting？这里用、which of。
which，英语单词，主要用作代词、限定词、形容词，作代词时意为“ 哪一个，哪一些；（指明事物）……的那个，……的那些；（进一步提供有关某事物的信息）那个，那些”，作限定词时意为“ 哪一个，哪一些；（指明事物）……的那个，……的那些。
（进一步提供有关某事物的信息）那个，那些”，作形容词时意为“哪一个，哪一些；无论哪个；（进一步提供有关某事物的信息）那个，那些”。
请注意以下几种只能使用which的情况:
1. 在“介词＋关系代词”结构中，关系代词必须用which。
如： The house in which we live is very large. 我们住的房子非常大。
This is the reference book of which the teacher is speaking. 这就是老师正在谈及的那本参考书。
注意： 如果介词不放在修饰事物的限定性定语从句的句首， which就可换为that， 例如： This is the question which/that we've had so much discussion about. =This is the question about which we've had so much discussion. 这就是我们已经多次讨论过的问题。
2. 先行词为“those＋表事物的复数名词”时， 关系代词通常只用which而不用that。
如：you should grasp well those skills which may be used in the future work.
最后，记得这个特殊情况吧：当先行词是表示原因或方式的名词时，可以用that引导定语从句，相当于介词+which，that可以省略。
如： She likes the child for the very reason that (=for which) she loves his father. 她喜欢那个孩子因为她爱着孩子的父亲。
He didn't like the way that (=in which) she speaks to his mother. 他不喜欢她对他的母亲那样说话。
Imagine the speed that (=at which) he drives the car. 很难想象，他开车开得那么快。
The reason (that /for which /why) he came here was to ask for our help.他来这里的原因是寻求我们的帮助。
