By girth

One system that has been suggested to me: organizing the books by the number of pages.

I don't think I'll try this myself, as 1) it doesn't necessarily make it easy to find books, and 2) I'm afraid it would be symmetrical in the same sort of way as organizing by colour. Of course, there would be some asymmetry - books with the same amount of numbered pages can have very different girth, if they are in hardback or have very thin pages etc. But still.