Translation:I am interested in books about birds.
If you're talking about interests in English, you usually express them as "I am interested in something". But in Spanish you construct the sentence closer to "Something interests me" ("Algo me interesa"), not needing any preposition.
- Me interesan los libros. - To me are interesting the books.
Interesar is used as a gustar-like verb here.
Part of the problem is that in native American english - we use the article to show a specific item. In spanish, they use the article to indicate a generalized group. So we use no articles and say, "I am interested in books about birds." We meaning we are interested in any books (or all) about any birds in general. If we said, "I am intested in THE book about THE birds." We would mean a specific book about a specific set of birds - for instance if a huge flock of birds had just taken over NY city, everyone would know at that time "THE birds" referred to that specific group of birds. In Spanish, the use of the article reverses the meaning and intention of the sentences.
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.