"Japan and the US Join International Design System", by Catherine Jewell (Communications Division, WIPO), has just been published in the May 2015 issue of the WIPO Magazine. It's probably the most readable and user-friendly piece of general literature on the subject to date.

This article doesn't contain any great revelations or arcane secrets about how the workings of the Hague System are affected by Japanese and US accession, or vice versa -- but it does give practitioners something that they can put in front of their clients and thus save themselves several minutes (or longer) trying to explain the significance of Hague and its new members to inquisitive clients. You can read it here.

If you visit the WIPO website you can also find versions of this article in French and Spanish.