I have a very, very strange way of arranging my books. I makes complete, utter, perfect sense in my head, and I'm not sure if I can explain it very well, but I'll try.
First thing to note is that I like to alphabetise things. Anything that can be alphabetisized, really. I used to volunteer in my local library and I loved to shelve the books when I had the time. Clearly, that's the most obvious item that can be alphabetised in a library, but I would do this to the DVDs too. One of the librarians thought I was crazy, but she did admit that it made it so much easier to find what she was looking for. Of course, I'd come back every week to find it messed up again, and so I would just have to rearrange it all over again.
Our DVD collection at home is alphabetisised too. My mum has done her books, because she didn't like the way that I did it. But my books? Nuh-uh. No way. When my housemates asked me how I arranged my books if not alphabetically, on finding out that I liked to alphabetise, my answer in a nutshell was: by how much I like the book. I'll now try and develop that a little further before I move on to the title of this post.
Order of categorisation for my physical bookshelves:
1) By Series - pretty simple: all books in a series will be grouped together. I did have this as number (3), but then I thought about it and bumped it up to first spot. This is because while (1) is the main way of categorising my books almost all the time, sometimes series will trump author. Most notably in the case of Buffy. The Buffy books and graphic novels that I own are all by different authors and are shelved together by series, not the author.
2) By Author - quite self-explanatory really: all the books by the same author are grouped together. I can't think of any author whose books are split up, except where some of their books are at home and some are with me in Brighton. Where I might have a lot of books by the same author (e.g. Meg Cabot) they might be split over a few shelves, but still on the same bookshelf in vaguely the same area.
3) By Genre. Ish - there's generally blocks of YA fiction, romance, paranormal romance, YA paranormal, fantasy etc. While not all of the 'blocks' are all together (i.e. ALL the YA fiction in one place) you can generally identify separate genres on one bookshelf. This is made easier by the fact that where I have multiple books by the same author, they tend to be in the same/similar genre, which means it keeps in line with (1).
4) By order of preference - basically, once (1) (2) and (3) are roughly grouped together, then they are organised on the shelf in the order that I like them. For example, I own around 40 Meg Cabot books. There're all together, in the sense that they are on the same bookshelf, but not necessarily on the same shelf because there just isn't room. Within that, all the books of the same series are arranged together and in chronological order. But then, when we move onto physically putting them on the shelf, I move left-to-right by the books that I like most, to the ones that I like the least. This principle also applies to books of the same genre but different author.
5) By practicality - sometimes, I just can't follow (4) as I would like because there just isn't room to. The majority of my books are shelved horizontally, so you can fit in more that way, but also by size too. So once (1) (2) and (3) are done, all the trade paperbacks will go together in order of (4) because this is the most practical way to maximise space.
That wasn't too hard, was it? It might not seem the most logical way to you, but it works for my brain and I love it.
I've never really been inclined to arrange my physical shelf alphabetically. I'm not sure why and I'm sure I'll try it some day, but I just don't want to. My Shelfari shelf, on the other hand (as you can get a glimpse of at the bottom of the page) is alphabetical because that's just the easiest way to arrange a non-physical shelf. As I mentioned above, the vast majority of my books are stacked horizontally and I like it this way as it breaks up the shelf and IMO makes it look prettier (because I'm clearly this shallow). Alphabetising would mean that everything would need to go vertical, and there just isn't room.
Then it occurred to me one day, shortly after watching this awesome video, that I should try colour-coding. I had been sitting at my desk trying to work on a group presentation (how I despise them) but the seed had been planted and its roots were spreading as I pondered. It would completely mess up the system I have just pain-stakingly detailed above where I know EXACTLY where every book is, even if I sometimes forget that I actually own the book. Colour-coding would mean that I would need to know the colour of the book/spine if I ever wanted to find it. Plus, I recently made a database of all the books I own which includes the field 'location' which would need to be changed if I were to colour-code.
The single argument put forward in favour of colour-coding was simply: think of how pretty it would look. I judge books by their cover all the time, and so this argument won me over easily.
I took a practice run on my books with me at uni to see how it would turn out. I've had a number of crazy stints where I've bought a ton of books and enough to try it out. Since taking the picture below, I've had more of these crazy stints, and so the picture does not accurately represent what my shelf looks like now. It might just be shallow me, but I think it looks quite good. I like how the colours flow into each other naturally and it would just be amazing on a large scale.
It hasn't quite confused my mind yet, largely because this is only a fraction of the number of books I have stuffed at home. Literally: stuffed. If I did this to every book in the house, not only would it take me days, but I wouldn't know where to find a book once I was done.
One thing it did take me a while to decide was the colour order. As you can see from the picture, the top shelf is white > yellow > orange > red > purple and the second shelf is green > blue > brown > black. Is this logical? If they were able to fit on one shelf, it couldn't be purple > green because purple > blue would make so much more sense. Green > yellow is logical, but then where does this put white? White > green > yellow > orange etc just seems plain strange. I'm not quite sure I could cope with this large-scale. And now it's clearly time to get my head examined.