That is an easy answer: I just let the books speak for themselves! At home, I fill our home library with REALLY great books. I read them aloud often and I think after doing that children will naturally grow to respond, recognize and respect great books. I think that even very young children will recognize a truly good story and good writting and will naturally be drawn to those types of books in the long term. Sure when they are young they may be temporarilly attracted to ‘twaddle’ but I think that is more because they are attracted to the characters of illustrations. I think that in the long term that a book about disney princesses won’t hold a candel to a Beatrix Potter story or a Hans Christian Anderson fairy tale. True great stories will hold their own ground and there isn’t really anything you have to do to help your child see that.
Also, I would like to point out as gently as possible that perhaps you should change the way you do things at the library. You say you give her a choice to pick out some books and then veto the vast majority of her choices. So see in reality, you aren’t really giving her much of a choice. For a moment imagine what it would feel like to be told that you can pick out some books and then have 3 out of 4 choices refused! At the very least, I would have to say it would be discoraging for the child. And for some children, it may even turn those types of books into “forbidden fruits” that makes them even more tempting.
So my advice is to either REALLY be prepared to give her some choice at the library or be more straightforward with her regarding your limits. Don’t put the sense of choice out there if you aren’t really prepared to give it. If you really want her to pick out a book, let her pick a book. (You know, within reason.) One twaddle book here or there is not going to ruin your child’s sense of taste. If you want to give her some more limits in her choices, then give her those limits ahead of time. Instead of just setting her free and then vetoing all of her choices, give her a choice between several books you approve. OR, set a limit in the number of books she is allowed to pick out. (Instead of four books of her choice, you could pick three and give her the complete freedom to pick one.)
I tend to think of twaddle the same way I think of sweets in my child’s diet. Sure I don’t want them eating cupcakes every day all day long. But a cupcake here or there is not going to ruin your child forever. Twaddle is the same way. No matter how hard you try, a child is very likely to be exposed to some twaddle books in their life. (At the library, at grandmas, at a friends, etc. etc.) The point is not to limit those completly, the point is what you do in the long run the majority of the time. And the other point is to let the good books speak for themselves. Your child is intelligent! Trust that they have the ability to one day decide for themselves what makes a good book a good book.