This is correct. Usually it’s the first letter of the author’s last name and a number that correlates with the publishing order of the publisher. Not 100% sure what the C on the つばさ文庫 is for. I don’t have letters on any other kadokawa books.
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You can actually see it on the picture linked above:
next to the feathers, it also has 小学上級から written.
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The Tsubasa Bunko of Suzumiya Haruhi I have has “日た”, so it’s not always a letter.
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You sure it’s 日 and not B? I just checked all my tsubasa ones and I have A’s, B’s and a whole lot of C’s but not any kanji. And wiki just helped me figure that out. A are originals, B are bestsellers and C are novelizations. 
(There are also D - nonfiction, E - famous foreign lit, F - famous jap. lit.)
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Yeah you are right, I had to look really close.
I guess then the letter is the genre.
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Hello, I’m also looking for the green labelled books the kadokawa page doesn’t work. Has anyone an alternative link maybe?
Looks like the page is currently under maintenance; they don’t give an end date, but presumably it’ll be back up in a day or two.
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Thank you.That would be nice. 
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Unfortunately it’s been that way for a while, apparently since a cyberattack in June, so it may be a while yet. In the meantime, searching on amazon.co.jp for つばさ文庫 may be the next best option.
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Not sure if this might help as it is in Japanese (you can take the isbn number of the book and search for it on your preferred site for buying from though).
There is also a list of similar providers here who also have furigana books.
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Thank you. 
It’s really a helpful site and lots of books. I appreciate it very much.
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No worries. Glad I could help 
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I have also found two sites with the collections with the blue and green labeled books…
Here are the links:
https://myfigurecollection.net/entry/130890
https://myfigurecollection.net/entry/105961
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