"제가 읽던 책은 두 권이에요."
Translation:There are two books that I have been reading.
That is definitely a more natural English sentence, but not a 100% accurate translation given the subject marker 은, making "books" the subject and the existing/to be verb of 있다. So the most basic idiomatic English sentence is "there are two books." The original sentence further modifies "books" with the phrase "(that) I have read." While the "correct" answer is an accurate direct translation, by far the best idiomatic translation is "There are two books that I have read."
I believe this is wrong but im no expert on this confusing grammatical concept.
I just read this: