At Mercury House I used to have a weakness for books that did not fall into clear categories. The most interesting work is often done at the margins, at the borders (which is also where translators do their thing). But books have to go somewhere in the bookstore, and publishers try to control there placement by inserting little category tags (usually placed near the price or bar code). There is an art to this, though a very imperfect one. On-line book sites face a similar problem. LibraryThing has compared their system of tagging to that of Amazon (guess whose they prefer) in an interesting post.