While I'll certainly agree that this particular guest was rude - I don't think that it's inherently rude to be interested in reading a book in the hosts' home, as a houseguest. People who love reading are often the first to notice a book that seems interesting to them, after all.
Yes, of course they should ask, if possible. But, if it's 2 am and they were attacked by insomnia and tiptoed downstairs to get a glass of water, I don't think they deserve to be cast into E-hell for picking up a book that they see on the shelf. If I was the host I probably wouldn't be too happy with a knock on my door saying "hey, it's Harry, do you mind if I read the copy of Persuasion that I found?" Just pick it up and mention it to me in the morning! Of course guests should bring along their own reading material, but sometimes a book is a quicker-read than they were anticipating, or they found that the book was too dull to continue reading.
Also, I don't think that anyone should be expected to automatically know which books belong to which partner. After all, when I still had Dh living here with me - anyone who assumed that the romance novels and the Jane Austen collection belonged to me would have been wrong....
Naturally, opinions differ greatly on topics like this. I remember a discussion on E-hell a few years ago that involved a teenager reading a magazine that he'd picked out of the magazine rack during a visit, accompanying his parents. But I'll confess that I can't remember who was upset - the parents, or the hosts.