It was such an interesting bookthat I wanted to read. Here the underlined clause is a/an -

 

Independent clause:
A clause in a complex sentence that can stand alone as a complete sentence.
In other words,
An independent clause is a clause that can stand on its own as a distinct sentence.
Examples: 
- I love eating cookies.
- My dog barks a lot. 

Dependent clause:
It needs an independent clause to finish or complete the sentence.   
Ex. When I got home, I ate dinner. 
Dependent clauses are sometimes known as subordinate clauses. Example: When I grow up

Source: Oxford Practice grammar and A passage to the English Language by S.M. Zakir Hussain.