No 100 books was the minimum, I lost count around the 100 mark.
The 3rd edition of PhotoReading has previewing for 3 to 8 minutes you'll find in the 4th edition previewing is 30 to 90 seconds. Preview was a quick glance of the front and back covers to make sure the books were of the genre I was as expected.
Any PhotoReader can do it. Go into a bookshop or library and start flipping.
Alex