On a recent run down to Tokyo, I had a peek at the revamped Yamano Music store in Ginza. They have managed to cram more ukulele material in their third-floor music book section than any music store I visited Hawaii--or anywhere else I have visited. Those in their Ginza shop can take the elevator or escalator to the third floor, go through the entrance to the floor room and look the the lower section of the bookshelves just next to ones to your immediate right.
Probably the only place in Japan to see more is to order it online from Gakufu Net.
Akio Instruments perhaps has the biggest seclection of English language Ukulele books in Tokyo that I know of. Click the button on the middle of their web page to get a pop-up map to his shop. If you are visiting Japan from overseas, it might be best to get the English books elsewhere. Some of the books there are a bit more than you would pay outside Japan.
Both Yamano and Akio sell ukulele and Hawiian CDs as well.