Generally this is a distraction to deep meditation. The purpose of the three-year retreat is to go beyond the books, having studied them for many years.
However, meditators may wish to review a topic they have studied and may read occasionally. Or they may wish to read a book as a way to relieve symptoms of lung, an inner wind condition that can occur while striving for deeper states of meditation than the body is prepared to go. But generally during the intense periods of retreat, meditators will not be looking to the outer world for answers.