Nihonjin no shiranai Nihongo (The Japanese the Japanese don't know)

Absolutely hilarious and educational book. Best seller in Japan. "The Japanese the Japanese Don't Know" is a collection of stories about a teacher of Japanese in Japan and her students, what sort of questions they ask, what sort of mistakes they make, etc. There's also a lot of interesting background on the Japanese language, like where certain words came from and such. For example, masu/desu is now standard Japanese, but it used to be geisha-speak. Men came to the capital, heard geisha speaking that way, and took it home and spread it, thinking it was the way everyone in the capital spoke. There was also a lot about keigo (formal speaking), and how even Japanese people get it wrong a lot. Highly recommended for intermediate/advanced Japanese students. All kanji with furigana reading.