I was talking about the books. I've been reading in the library: This version features two independent phrases that, although related, show two distinct actions of the speaker. First, the speaker is commenting on the fact that he is talking about books and that he was reading in the library.
I was talking about the books because I had read in the library: This version of the sentence was created in a cause and effect structure that shows that the speaker was talking about books because he was reading in the library, which gave him enough knowledge to speak
