By the year 2008, Harry Potter, J.K. Rowlings' gripping book

The basic sentence is:

Harry Potter had around 200 million copies in print.

If you add modifiers you cannot change the structure, let me explain with an example.
One of the most succesful book in our generation, Harry Potter had around 200 million copies in print.

That part is a modifier correctly used that does not affect the core sentence.

If however I say:

Harry Potter, which had around 200 million copies in print. This is not a sentence, because if you remove the "which" modifier you are left with "Harry Potter": not a complete sentence.

Same thing with E:
By the year 2008(modifier), Harry Potter (subject), J.K. Rowlings' gripping book series portraying a young wizard's struggle against the evil wizard Lord Voldemort (...) book ever written (this is a description of the book).

If we want to be precise, those are called appositive modifiers, they give a description of a word like in this case with "Harry Potter", and in this case the commas act as boundaries.

The first comma starts a description of the book that never ends, so the structure of the sentence is : modifier, subject, modifier.

A run on sentence is a sentence composed by two (or more) indipendent caluses connected by a comma without FANBOYS (and, but,...)
Example: IC 1= This year I'll go to Paris ; IC 2=Next year I'll go to London
Run on sentence=This year I'll go to Paris, next year I'll go to London
Correct sentence=This year I'll go to Paris, AND next year I'll go to London
Hope it's clear