Consider allowing "I need a library book." As an acceptable translation.
In engilish a "library book" is considered a "book from the library"
I think that would be 'libro de biblioteca'.
If the sentence had been '... desde la biblioteca' I think it would have caused less confusion.
You are correct, "I need a library book" does translate as "Necesito un libro de la biblioteca."
Accepted July 25, 2018.
This should be THE correct answer, and the other an alternate.
Duplingo needs to be more accepting of the small pronouns like en y de, one time de means of y another it means from. How are we supposed to learn if they don't accept normal conversation?
Jimi, I agree many phrases have slight differences in prepositi use that are "normal conversation," but in this case, if you had to translate this example to SPEAK the phrase, saying "I need a book OF the library" would not sound natural. Hope that's helpful. :<)