One thing I've run into over and over again through the years is the fact that I will rarely have enough shelving to put all my books on at once.
I did manage it for a while in Canberra, but only because the interior walls of the house were effectively plated with cheap freestanding bookcases. I think at last count there were over a dozen, each with just over ten feet of linear space. (And of course there's double-stacking, and sideways storing, and all those tricks.)
Those bookcases were discarded in the interstate move as being too bulky (and non-collapsible) to be worth the moving cost. Since then, many cheap and free bookcases have found their way into the new place, to the point where
megpie71 actually has a tiny bit of unused shelving. Me, I still have cardboard boxes full of books which haven't seen daylight for a year or more. I'm half tempted to convert one wall of my walk-in-robe to shallow bookshelves, as I never use the space for its original purpose anyway. Hmm... let's see, that would give me, what, about fifty linear feet of shelf?
I'm only an occasional accumulator of books, so that should hold me at least until I get serious about really addressing the shortcomings of my library at some future point when I have the time, money and willpower to reindex them and spend a day or two on Amazon abusing their "people who bought X also bought Y" feature.
That'd be an interesting little computer program, though. Feed it your book collection database from any number of online or offline sources, and it would compile the complete set of "Amazon customers recommend" results and present them to you sortable by title, author, publisher, number of books you have which are a single DB link away from it, ISBN, and possibly also the best price it's available at from Amazon and a number of other internet retailers of your choice. Export all or part of the result in a number of formats, including spreadsheet, CSV, HTML, various blog formats, and birthday want-list.