A bind-up, though it sounds like some interesting minor S&M practice, is a bundling of several books into a single binding. If you have published several books in a series or on a similar theme, and some time has passed and sales are starting to flag, the publisher might propose a bind-up. Go for it! All the little books will be gathered together and forced to be friends with each other in a single volume. There will probably be a new cover design, the books will be given a new, updated look, and the aim will be to attract new readers who were possibly not even born at the time the originals were published. Remember - foetuses don't read. The picture shows the bind-up of Kath Langrish's hugely successful Troll trilogy, revised and reissued in a single volume, (March 2011).
A bind-up is a way of making extra money out of a bunch of books that have slipped from view. Bookshops don't often stock many books that have been around for a few years, unless they are classics, steady sellers, or by major figures who always attract a lot of readers. Don't expect to see a bunch of Jacqueline Wilson novels appearing in a bind-up, for instance - they are always in stock and sell anyway - though I think there has been a Tracy Beaker bind-up.... A bind-up is money for old rope. You don't have to do much, and the publisher doesn't have to invest much, and it's a lot less aggro than writing and publishing a new book.
And here's the opposite of a bind-up.... Lucy Coats has extracted 12 slim volumes from her Atticus the Storyteller, each dealing with a separate Greek myth. The slim volumes have bright, colourful covers, they are easier to carry around and hold in small hands than the original and are less daunting to unconfident readers.
So bind-up or bind-down, there is potential for rebadging books and gathering new readers either way.