Slipcases. Some other publishers have done something similar - e.g. Trudvang. This was pretty much what some of us settled on when they first announced it. It isn't all that different from a box set, but disallows non-paper elements, but comes with a sturdier case and still gives that "books-on-a-shelf vibe."
I'm also guessing that this is a bit of a test product, that we might see Planescape and/or Dark Sun published like this. The downside is that it less of a general setting, and more focused - so we might see a Tyr Dark Sun set, or Sigil for Planescape - but not the wider settings.