Translation:I am interested in books about birds.
I'm still trying to get the hang of when to use articles and when not to. Because "me gustan las bananas" uses an article for the banana, "el libro sobre pájaros" doesn't use an article for the pájaro, "ella estudiar medicina" also doesn't use an article for the medicine... I guess I'll have to memorize each types of sentences but it'll really help to have a framework.
To those debating whether it's correct to write, "...books 'about' birds," or, "...books 'on' birds," the correct preposition to use in English is, "about" (i.e., "...books 'on' birds is NOT correct English). The preposition "on" refers to location (e.g., the books are "on" the table). Thus when Duo Lingo hasn't accepted "...books 'on' birds," it's because it is indeed incorrect.
Books about birds interest me. (Plural subject with matching verb)
This book interests me. (Singular subject with matching verb)
But in English we typically put this sentence in the opposite order, so to make the sentence sound more natural in English, we don't do a direct word for word translation.