Since 2008, we've offered a small feature to download printed books from Wikipedia article. This is done in partnership with a company called PediaPress.
They've sold about 15K books over that time period, not enough to break even, and the support/maintenance burden for the service is no longer worth it for them.
We'll disable this feature in coming weeks. We'd only continue to offer it if there's 1) strong community interest in maintaining it, and 2) a partner who steps up to provide the service.
We'll continue to provide PDF downloads (soon with a new rendering engine).
Thanks, Erik