I realise this is a slightly older thread but thought I could offer some ideas
I assume tokyo has waldorf schools since they appear to be absolutely everywhere so that might be a great start. If not myriad toys would be a good place to see books that are generally in a more natural setting though some can have Christian themes.
I am linking myriad only because the books are all together easily they should be available from amazon etc too http://www.myriadonline.co.uk/books-for ... beskow.php
Elsa beskow is fairytalish and very lovely imo she has lots of seasonal stories without any religious slant to ithttp://www.myriadonline.co.uk/books-poe ... ildren.php
Stories of books are again very lovely they feature mini people that represent certain aspects of nature (ie flower fairies mother nature king winter etc)http://www.tiptoes-lightly.net/
wonderful books that are low on pictures. All heavy on nature lots of nature apreciation. Main character is Tip toes a fairy who has 2 gnome friends and was born from the sun and the wind.
I realise they are not druid stories but all are so nature based I find them very suitable