We investigate the practical short-term and long-term effects of five different frequency ranks used for selecting which out-ofvocabulary (OOV) words to add to a pronunciation lexicon for text-to-speech (TTS) of university textbooks. The work is an empirical study on a corpus of 200 university text books selected for talking book production and it takes the extensive pronunciation lexicon of a commercial text-to-speech system as its baseline. The main take-home message is a short but succinct set of guidelines that promise to increase the efficiency of OOV management, at least for text-to-speech production of university text books.