I asked earlier if we could PR while watching TV (looking over the book at the tv while still able to see all 4 corners) and no one said anything. I think the mantra's and "not thinking about your day" ideas are about mental focus similar to what they do in yoga or martial arts classes where they tell you to let go of all nagging thoughts and come back to the present moment.
If you're going to read many books on the same topic (for syntopic reading) then you can use the same purpose for all of them and go right thru them. If you're going from one unrelated book to another you do have to stop and do the closing affirmations and then with the other book start with the opening affirmations and affirm a new purpose but I don't think you really have to wait any time to do that. The 20 minute wait is for activating a book you PR'd not time between Photoreads.
Syntopic reading is incredible. If you think reading 1 book in 45 minutes is good try hitting 5 at once (obviously all on the same topic). I don't know why so little time is given to it in the book and it's not on the tapes but for some reason doing it changes your thinking. Reading 1 book makes you think of the book but reading 5 books makes you think more of the topic. It's so weird, you have to try it.