I'd love it if any of my boyfriends friends were interested in my bookshelf when they came round to our home and if they borrowed something I'd find it such a compliment! Unfortunately when people come over, the only thing that generates any interest is the Play Station 3.
I wouldn't mind someone coming into my home and without asking pulling a book from my bookshelf to read or a film from our DVD collection to watch if they wanted to but then again, if I have a guest round, I subscribe to the 'my home is my guests home'. I totally get that people might find it uncomfortable with guests examining and touching their things without asking but just in case, it's always better to be polite and ask to borrow something without just taking it as a general rule. If I wanted my books to be kept in good condition (which is a terribly sensible idea) then I would ask my guest if they wouldn't mind just being careful with it.
I think things like books are a little different to other things and 'fair game' to be looked at and touched but I don't think I'm going to do a good job of explaining why. I just see books as such a good conversational point in life. They're spiritedly and heatedly debated over, people cry over them, laugh over them, fall in love over them, live their lives by them, study them, adapt them - books are truly the best friend one can have! I would think it's normal for a guest to look through the books to discover your tastes and find out more about their hosts. My library collection is full of classic novels and I feel my books are an extension of myself of which I would love my guests to want to get to know more. As long as someone I hardly know doesn't say "Wow! you have Middlemarch here, you're such a loser!" I wouldn't mind a little light-ribbing".
And just to settle a side-moot, I too agree that a guest of my s/o is as much my guest. Despite how little or how much I know him, I always welcome him with open arms and free-flowing beer! unless he's Chris. I hate Chris.