During a break a little while ago, I found this article on the Bookshelf as Identity. I was reminded of an article a friend sent a while back, entitled The Library of Congress Comes Home. Both articles are about bookshelf organization, though from very different takes. And both concentrate on the topics of the books.
My bookshelves are topically organized only in very broad strokes. Books that have something to do with religion or philosophy mostly go on one shelf -- though that shelf also has my copy of The Smithsonian Book of Books and The Complete Guide to Calligraphy. I would say that this indicates something about my personal pantheon, except that the same shelf has Fermat's Enigma, Stairway Walks in San Francisco, The Pig Who Sang to the Moon, and Worms Eat My Garbage. Further down is a shelf which houses a few history books: From Dawn to Decadence, Africa in History, and The Soong Dynasty are there. So is Autumn Lightning, which is half history (the other half is autobiography). But wedged right next to those books are Kay's Tigana (fantasy), Zinnser's On Writing Well (what it sounds like), Brooks' The Mythical Man Month (software project management), Icke's Force of Symmetry (physics), Asimov's Azazel (humor), and an odd copy of SIAM Review (an applied math journal). Furthermore, Churchill's History of the English Speaking Peoples and Lewis's The Middle East are a couple shelves down; and Herotodus is again on the shelf with philosophy and religion, two books down from the Tao Te Ching and three from Marcus Aurelius.
So what's the correlation? Well, if you took all the books from one of my shelf and put them into a cardboard box, you'd probably find that they packed together pretty well. The philosophy shelf also happens to have a lot of books that are shaped oddly, and didn't fit that well with anything else -- the Book of Books in particular is very tall, and doesn't fit at all onto some shelves. Most of my science fiction fits neatly on the bookshelf by my bed, simply because those books are all paperbacks of roughly the same height, and taller books don't fit well on those shelves.
There's some shuffling over time, of course, as I take a book down for reading or simply for reference, and as I put back books that begin to clutter my desk. And the books in my office are a little more topically organized: mathematical analysis in one place, computer systems in another, mechanics in another, and linear algebra -- well, linear algebra moves about the shelves and about the office, but there's some sort of concentration between the books on technical writing and the books on analysis. Even there, though, I organize by physical shape and by the frequency with which I refer to things before I organize by topic. And it shows: there is no topical reason for the top-shelf mixture of books on statistics, asymptotics, special functions, and other amusements, except that a lot of them are Dover paperbacks with sort of similar shapes.
It's fortunate that my spatial memory is better than my temporal memory, or I'd lose all sense of where my books live. But if you spoke no English and were somehow tasked with finding a place for a stray tome from my collection, I suspect you would manage to find a good place for it. Just be careful if that place is the shelf o' philosophy. You might knock over the origami cat.