To sum it up I expect to be 2KYUU level by the end of the year . I could work more to get 1KYUU next year but I have something else on my mind : thanks to my school I'm going to enroll in a japanese university (waseda or tohoku according to my level) and then start an internship in a japanese company for 6 months . So it seems to me that the next step ,as something unavoidable for anyone willing to work in a japaneses company, is to really improve my keigo.
So do anyone know about a reference for this complex but sensitive problem ? japanese themselves have a hard time with it so I guess a specific training for several months could be useful to be able to be operative in my work.

Read about this book 新版人に聞いてもわからない日本語知識辞典 but the scope seems a lot larger than mere keigo ....it's more like a book to litterate native-like fluency .
And I'm more looking for a book dealing exclusively with keigo .

Don't know any off the top of my head. What might work though is if you go for a book on business Japanese, which will automatically feature a lot of keigo.

Keigo isn't too difficult to understand once you've got the idea, it's just difficult to produce. There are set phrases that help a lot like ～させていただきます and so forth. I took a class in business Japanese that I found valuable.

Just to add, to hear keigo, watch a dorama called atsuhime. It is allmost only keigo.

I didn't check them myself, but I know they exist :
http://www.amazon.co.jp/気持ちが伝わる敬語の使い方-小崎...6RZKSV5LNC

http://www.amazon.co.jp/好感度120%25UPの敬語と話...6RZKSV5LNC

Both seem what you need.

P.S. The second book has a preview on Amazon...

I own this one:

http://www.amazon.com/Keigo-Modern-Japan...213&sr=8-1

While more of a linguistic analysis, there are many sample sentences. Provides a good foundation of origins and today's usage - as well as Japanese natives take on keigo in general.

-d