The sentence given above is A COMPOUND COMPLEX SENTENCE.
A compound complex sentence is a sentence which is made up of at least two independent clauses joined by a conjunction and one dependent clause. In the sentence given above, there are two independent clauses and one depend clause as follows:
1. The first three Harry Potter books landed on the bestseller lists.
2. Both young and adult enjoy the stories.
3. That follow Harry through his challenging life.
Sentence 1 and 2 are independent clauses while sentence 3 is dependent clause.
A. To show the reader that the writer is knowledgable
hope I helped
