Thanks for the site, it is a very interesting concept!
By the way, I considered sending this to you via email, but decided against it to allow others to (a) see what I already reported, and (b) comment and improve on my ideas if they like. I folded my thoughts away so it doesn’t take up so much visual space, though…
(If you would prefer me to send you an email instead, please tell me and I’ll comply)
Some Natively issues regarding editions
I’m pretty sure you’ll see 
The cover images of vol. 1-3 are from a different edition than the cover image of vol. 4. See the green border around the image? This indicates the full-furigana edition. Here are the non-furigana images for vol. 1-4:
If it’s an edition with more vs. less furigana, that would immediately feed back into the perceived difficulty for me, though. (By the way, Kiki has the same issue, there are two editions with different furigana amounts.)
Also, once people start venturing into older books, you might have books with old vs. new kanji/kana/grammar as well which all of a sudden could make all the difference between “nice read” and “unintelligible” 
Also please note that the 鹿の王 as well as the 獣の奏者 series actually consist of five books (the fifth one of each does not have the number 5, though…). Oh and I think (but not sure really) that the full-furigana children’s edition of 獣の奏者 even split each book into two… 
For Fruits Basket, it seems you are listing the Collector’s Edition (judging by the covers). That one groups two volumes of the original edition into one which made selecting them a bit confusing for me… My suggestion would be to at least indicate “Collector’s Edition” in the title so that it becomes clear to the users which edition is being listed.
No issue, just a remark: “The Perfect Insider” is the first in a series of 10 novels.
Some thoughts around displaying series
I really like to look at the search page and to skim all the books, and it’s very nice that directly below each book I can immediately see whether I already added it to my list - if it’s a standalone book, that is… For book series, I have no visual feedback whatsoever. I cannot even see how many books you currently have in a given series. Plus, checking a series means opening a new page. This makes dealing with series a bit cumbersome for me. Therefore I have a couple of ideas:
- display below the book series how many books are in it
- indicate below the book series how many of these books are in which category of mine
- make the books in the series more easily viewable, without opening a new page. I really like the way bookwalker displays series in their “what you did not yet buy from each series” list. Initially, each series is displayed as just one entry, pretty much like learnnatively does it (plus it indicates the number of entries):
If I click on the arrows icon in the bottom right corner, I get all the members of the series in that same page:
If that list is too long to fit in one row, I can scroll left and right. This way, it always covers just one row. (If I expand another series, this one gets closed first.)
This way, I can operate on all the entries of that series on the same page, without the need of opening a new page and clicking around etc.
Speaking of which, I encountered a bit of a display bug: I marked a book as “finished”, then switched to looking at a series, and when I returned, that book was visually unmarked again. When I reloaded the page, the book was marked as “finished” again. So the backend had received the information initially, just the frontend had forgotten about it in the meantime…
Some ideas for the book page
On each book’s page, you list each grading which is very interesting! But it takes up a lot of space:
- It is unnecessary to list the name of the book in each row. It would be fine to have “User graded as more difficult than Other Book” just in one row.
- I would find it much more informative if the gradings were sorted by the book being compared to, so that I can see whether many others agree or disagree with my grading (so I can maybe think about my own judgment once more).
- If the gradings are sorted by the book being compared to, it would be sufficient to have just three entries (easier, same, harder) per “other book”, with a list (foldable) of users who gave that grading. (Think of having hundreds of gradings at some point…). You could also maybe indicate the percentages of the votes (“33% of graders think that this is harder than Other Book”).
- If we only have three entries, it would be helpful to sort by “percentage of votes”, so that e.g. the easiest other book that was compared comes first, and the hardest other one that was compared comes last. Again, this would help me compare my own ranking (that I have in my head) to the list.
I hope I didn’t scare you too much with this long list of ideas and suggestions Please keep up the good work!
. I’ll follow your lead with folding thoughts, good idea
Good job Mr. Detective 
I also briefly thought about hovers but it’s more hassle to operate even on desktop in my opinion. Glad you like the suggestion!
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. If that’s the case, then I’ll probably leave it as is and analyze the issue later. For those exceptional cases, I could always just make two books with some warnings in the description and call it a day.
No limit, only as many as you care to add! If you want to add them all, i’d absolutely love that. I will say though it may break the general difficulty grader, I haven’t stress tested it with that many books