Books that can be turned in have a value of 1, a green cover, and show a special "Book" icon, while Wolfgang's books have a value of 5, a blue cover, appear with the "Junk" icon in your inventory and are not accepted by Scribe Yearling. These "false" pre-War books can be found throughout the wasteland.
She does not consider a sheet music book to be a "true" book for her purposes, nor will she accept skill books of any kind.
Technically this quest is never-ending, but the amount of books you can give is limited to the amount of Pre-War books available in the game. Meaning even if you've found and given every book there is, the quest will not be completed.
pcps3xbox360 Sometimes, Scribe Yearling will ask you how your search for the books is going, and when you are given the dialogue choices with her, you will not be able to give her your pre-War books. [verified]
PC users may be able to work around this bug by selecting Scribe Yearling in the console and typing startconversation 14 FFDCArlingtonScribeYearlingGreeting1.