Tae Kims "Learning Japanese" offers a superbly simple deconstruction of a complex language into its fundamental components through a series of easy to understand lessons.
Time and again you will find yourself exclaiming "a-ha! (i.e. でさえ、ですら、ーかねる、~ざるを得ない、だらけ、まみれ、etc) as the subject matter and the method of teaching fit together seamlessly.
While "Learning Japanese" might not be the equivalent of E.B. Whites "The Elements of Style" for students of Japanese, the application certainly solidifies the unstable ground walked on by many whove struggled with the language over the years.
If given the chance to improve an already incredible app, I would build a companion app covering much of the same material and deploy it with a game mechanic that challenges students to put their new knowledge to the test.
Id pay $10 for that.