that there is??是定语从句吗

This is the best book that there is on the subject.
This is the best book where the book is on the subject.

where = that there, 这里用that there因为最高级的问题。

如果硬要还原，就是：this book is No.1 on this subject. No.1就是where了。

定语从句, 修饰book.
可以直接还原成This is the best book on the subject.
这是该学科最好的一本书.

应该是状语重句吧？
翻译过来，应该是：
在这门学科上面，它是最棒的一本书。