There has been a suggestion to require difficulty level tags with each review, and I think it is a great idea. Let me know your thoughts about these levels...if necessary I can adjust them depending on your input.
It has been suggested to include more gradations for easy books, since people just starting reading in Japanese might be more dependent on book recs than people who are more advanced readers and can handle just about anything. So I am splitting up easy books by approximate grade level.
NEW EDIT: When thinking about these levels, instead of interpreting them as "how difficult was the book to read for you", please think of them as "how difficult would the book be to read for a Japanese language learner whose Japanese may not be perfect". Your kanji skills may be awesome so it was a quick read for you, but what if someone had to struggle a bit more with kanji, would it still be a quick read?
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Beginning Early elementary grades. These are often all kana or might include just a few kanji with furigana. Also includes beginning-level texts prepared especially for Japanese-as-a-second-language students. Think Japanese equivalent of "See Spot Run" type readings that use simplified language for the purpose of teaching reading skills to beginning readers.
Very easy Upper elementary level "chapter book" level readings. More complex vocabulary and greater length than beginning level texts, but furigana on most or all kanji. Often includes pictures. Kaoru Hayamine's books (e.g. the detective series Meitantei Yumemizu Kiyoshiro) would fit in this category.
Quick read (Young Adult/teen reading level). A person who has passed JLPT Level 2 or 1 could expect to read one of these without too much trouble. I would put Yamada Yusuke's books at this level. Some of Higashino Keigo's shorter books would be too, but I would put his longer works like Byakuyakou in Moderate Difficulty for greater length and complexity.
Moderate difficulty (a normal reading level for Japanese adults). This is just "medium" difficulty for people with experience reading Japanese novels who can read a large number of kanji. A person who has passed JLPT level 1 could expect to read at this level, maybe using a dictionary sometimes, but you would be able to follow what was going on in the plot without too much confusion. I would put Kishi Yusuke's works at this level.
Difficult Longer, more dense or hard to follow than normal prose. May require more effort and/or more use of a dictionary. Don't be afraid or embarrassed to put books in this category--we want to know if it's difficult!! If it had you thinking "what is going on?!" while you were reading, and you are usually a good reader, it should probably go in this category.
Extra-difficult Uses archaic language, and would be difficult for native speakers.
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There is probably some overlap between these and some room for subjectivity, but I thought we might want to have more levels than just easy-medium-hard. So if you think these are OK, would you please go back to your old reviews and give them a difficulty level tag? And tag new reviews with these from now on? This will make it easier for community members to find books at a suitable reading level for them. Thank you!!!
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EDIT: I don't think we have any "beginning" level texts reviewed here yet, so if you have read one or more at this level and you want to review them, please post your reviews, with the "beginning" tag! That will be a big help to members who want a good manageable starting point to reading in Japanese. Thank you!
- *New* Difficulty Level Tags - Edited, please note changes!


And I would call 黒い雨 Difficult. It is both old and literary:)
But the final call is up to you:)
点と線 may be borderline between quick read and moderate difficulty, because it does have a lot of dialogue and is short. But even in the first few sentences it uses rather sophisticated vocabulary like 某省and long strings of kanji like 機械工具商安田商会. I just think someone might stumble over those if they are not used to it, for example non-native speakers who are used to reading more contemporary authors. I think you are very good at kanji so maybe you weren't thinking about it from the perspective of someone who has to try a little harder to read kanji. I guess the point about these levels is not "how difficult was the book to read for you" but "how difficult would the book be to read for a Japanese language learner whose Japanese may not be perfect". But I left the "yomiyasui" tag in, so that preserves your impression that the book was easy to read for you.
Oh, while we're on the subject of tags, we have "akutagawa prize" but maybe we should have some other tags for other literary awards? Quite a few of the SF books I've reviewed have been either Seiun or Nihon SF Taishō award winners. I don't know whether we should have a tag per award, or just a catch-all 'award-winning' tag...
I agree, and that ought to be a good rule of thumb. I also agree young adult/light novels might go in moderate if they have convoluted grammar or are a bit hard to follow, etc. We can't always tell by the genre how difficult that particular book will be.
I will add Seiun and Nihon SF Taisho as you suggested! Great idea! I'll hit up Wikipedia for the names of some other awards.