Sentence:  <WSD>Book words</WSD>—words that are found most often in writing but are rarely spoken—change as well but in a somewhat different way. When the communal memory of a word's pronunciation has been lost, or when speakers who have never heard a <WSD>book word</WSD> pronounced attempt it, then spelling will play a key role in determining the new pronunciation. 
Meaning of the word marked with <WSD> tags:  A word that is encountered in writing but not usually in speech, which people may therefore not know how to pronounce (correctly).