On Anna's recommendation, I bought a copy of The Sewing Machine Attachment Handbook, which turns out to have been the book I've been looking for all my sewing life. Did your sewing machine come with a box of tiny, weirdly-shaped feet? Then you need this book. There are clear instructions for guides, hemmers, rufflers, binding and cording feet, sequin feet, walking feet … and on and on. If you can screw, bolt, strap or clip it to your sewing machine, I think this book covers it.
My favorite guideline for judging sewing books is by whether or not they make you want to sew — and this book makes me really, really want to sew. I think the hemming foot instructions alone are going to make my sewing life better.
It's also kindled in me a deep and painful longing for something called a "bias cutting guide" — a doodad that clips to the end of your scissors so that you can cut bias tape without all that painful marking. I wants one, I do. Couldn't turn one up on eBay, though; does anyone have any leads?
(Also, if you buy it from the link above, the author, Charlene Phillips, will sign your copy. That's always nice!)