You wouldn't need to preview a book you PhotoRead before.
Reason you don't want to spend too much time in previewing is it becomes too easy to start passively reading and before you know it you've wasted 30 minutes. Then it can hinder you during activation by limiting your focus on what you've already previewed.
If you happen to have a book you've PhotoRead before no preview at all is required. Allow the PhotoReading step to help you remember waht the book was about.
Alex
