Contrary to above information,
I cannot find Japanese books on Yutopian
I purchased the books from Katherine Yuan of Yutopian at the US Go Congress. The books do not appear to be listed in the website. I would call her (1-310-578-7181) and ask for the books. Each time I have called here in the past, she has been very helpful.
While speaking with her, please ask about the Speed Baduk for Kyu Players? series published by Oromedia. Two other sets (Speed Baduk for Beginners and Train like a Pro) are too easy or too hard respectively.