This topic is discussed in "When there's a Will, there's an A." Optimally, do not combine subjects. Activate a book on one subject to the point where you have what you need before you PhotoRead another book. Obvious exceptions are when dealing with school textbooks. It's best to PhotoRead ALL the books before activating ANY of the books. Also, when dealing with multiple books on the same subject, there is a cumulative value in PhotoReading them all at once.