Right now, it’s only available in Japan, but series creator Kotaro Uchikoshi said he believes an English version will “be released by autumn.”
The puzzle bits have been removed from this version, which doesn’t bother me, since the puzzles in 999 weren’t that great. Virtue's Last Reward vastly improved them. Plus, the iOS version also includes a flowchart of the game’s diverging storylines, one of the most useful additions to the sequel.
A year ago, I’d have scoffed at the term “visual novel.” 999 and VLR won me over with its batshit crazy but surprisingly logical (!) universe. Even though VLR was released on 3DS and Vita, you should really play 999 in order to enjoy VLR's payoff.