What is the most oddly specific topic/subgenre that can be found in your book collection?

I've always enjoyed browsing friend's bookshelves whenever I get the chance because it's like a mini-window into their interests. The last time I was reorganizing my own shelves I was a little surprised to realize that out of the general genres I had minisections for animal cognition, unusual YA perspectives on WWII, and several vampire anthologies. Have you ever found yourself collecting a very specific kind of book without realizing it?