Leaving aside the commenting, which I think is rude, I would have asked you if I could read the book, but I would also assume that you would say yes and that asking was just a formality. When I've stayed at people's homes, I've always assumed that bookshelves that are in common areas are "fair game," in that if I wake up in the middle of the night and can't fall back asleep, it would be okay for me to select a book from the shelf and read it, without asking permission. I also would never fold a page or damage the book in any other way.
I think there is a big difference between picking up a book and fiddling around with other things on display like a figurine. Books are meant to be picked up and read. Well, unless you have a rare first edition of Don Quixote, or something like that, but then I wouldn't leave that stuck in a book shelf out in the open...
I love books and I love to read, usually reading 3-4 books at once, each in a different room in the house. I rarely lend my books out of the house, but will happily lend them to a houseguest.