Well if the book still looks brand new and the recipient really has no way of knowing, then I can't really call it rude.
I don't think this would bother me at all, actually.
My sister is notorious for "Hey, I have your birthday present. I'll get it to you as soon as I'm done reading it." (She really does pick out books she knows I want or will really like. We just happen to have similar tastes.) It doesn't bother me when she does it because I know she wouldn't mind if I did it to her. I also don't really like lending my books out, so if a friend wants to read the book they're giving me, I'd rather they read it before giving it to me.
I'm kind of torn on whether I would ever do it, though.
I probably would with my mom or my sister since I know they wouldn't care. I could be up front about it and they wouldn't think any less of their gift. I don't think I would do it with someone I felt like I needed to be discreet about it with, though. There's something disingenuous about presenting a gift as brand new when it's not. If I wasn't positive that the recipient would be fine with my reading it first, I wouldn't. Even though in all honesty it does seem like a silly thing to be bothered by for the reason Julia mentioned.