I'm probably the last one to notice, but since I spent quite some time in bookstores today, I really suffered from this: Because books are relatively thin the title and the author's name is often written vertically (rotated 90° anticlockwise) on the spine.
So far so good.
Now most bookstores fill their bookshelves by placing book next to book with the spine towards the room. And this, dear reader, is where the problem arises. Due to the fact that the title is printed vertically, one has to lean the head by 90° in order to read the title when skimming through the shelves looking for a book. After a few minutes this really starts to hurt, and the eyes of human beings are not really made to work in this position.
Sure, one can't expect bookstore-owners to arrange all books with the cover towards the room, but at least having a working index that helps people finding what they are looking for might be a good start.
Thank god it's Christmas.