Ha! I love this question. It's like when I make soup for the kids and ask them later what did you have for dinner, and they said "We ate soup", we all have a good laugh, because they actually "drank" soup.
I personally think it's okay to say that you read the book because the person who you are talking to will know that the book in question does not have any words. But I am not sure what is the 'correct' way of answering this question.
Although it is not technically accurate, there isn't really a better way to describe it. To say you 'looked' at the book, suggests that you didn't go through it. While the definition of 'read' is the interpretation of text, you have phrases like 'read between the lines', and 'read into' - Reading is more about visual interpretation than necessarily having words.