User blog comment:Kaido King of the Beasts/Chapter 840: My Worthless Son/@comment-24442815-20160922021407

Someone pointed out on another site that the books next to Sanji on page 16 are Noland the Liar and The Devil Fruit Encyclopedia. Sanji mentioned reading about Noland the Liar back in Jaya and he knew about the Suke Suke no Mi from the very encyclopedia next to him on that page. I guess the reason he wanted to turn invisible in the first place runs deeper than "to peep on girls".