Thanks for all the suggestions. What I'm going to do is: 1. Any books that would not "curve" if set upright will go on a section of our bookshelves. 2. After culling the sheet music, all the random 8 1/2 by 11" photocopied stuff, and soft, thin books, I will look for a used, tall filing cabinet, or two two-drawer cabinets, and spray paint it/them a burgundy/cranberry red. (Good accent color in the living room.) 3. Then I'll keep the music in hanging folders, possibly color-coded for each instrument, or at least labelled clearly. Anything really odd-sized that won't fit in the hanging folders can go in some sort of labelled magazine file or flat storage box on the bookshelf. I don't have a tablet, and probably will never get into a digital system of storing all the music in my computer and tying in with a big flatscreen monitor on the music rack of the piano, as cool as that might be.
If serendipity happens, and I come across some kind of gorgeous, antique wooden music cabinet that somebody set on the curb, of course I might go that route. But I think a filing cabinet/s will be easier to find, and easier to customize. The living room near the piano has perfect empty corners either for one tall, or two two-drawer filing cabinets, if they're customized to look nicer than the generic blah-beige ones.