With your Tokyo address you can go wild with Amazon Japan, which has great delivery service. Book Offs are wonderful if you have oodles of time to browse. Do not skip the magazine sections, where you'd probably find lots of Ringo stuff for 100 yen.
The most sophisticated book on Ringo is probably Casio Abe's
Shiina Ringo vs. J-POP. Abe is a cultural theorist, so it'd be pretty heavy going, language-wise.
http://www.amazon.co.jp/%E6%A4%8E%E5...9E%97%E6%AA%8E
For a fan-oriented work, can't go wrong with
Apple Complex.
http://www.amazon.co.jp/%E6%9E%97%E6...6TV1G6M27A34WF
Unfortunately there isn't much of substance about her Jihen, post-Jihen stuff, apart from magazine articles.