My suggestion is add two genres: Novel and Artbook.
In database we meet works of this sort, but in many case we don't know that.
For example:
http://www.mangaupdates.com/series.html?id=4970
http://www.mangaupdates.com/series.html?id=5706
http://www.mangaupdates.com/series.html?id=4971
this is novels, but from info what we have can you tell that this is not manga?

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Lol... when I read this suggestion I though... wow that's so true... As I was thinking of Full Metal Panic! and Suzumia Haruri no Yuutsu.
But it seems that wasn't exactly what you had in mind when thinking of this!
Anyway I fully agree, a Novel genre might be needed but the Artbook, not so sure about it...
Aren't Artbook always related to a series, like an Omake or something?

This certainly makes sense, even though the examples you provided are all yaoi/shounen ai ^__^
But we have other examples for novels, such as Suzumiya Haruhi the novel.

Anyway, it's nice to see a fellow Lithuanian around here, so welcome and enjoy your stay.

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Are these really genres... or maybe really just categorizations or something. I think the genre section is getting used a bit too much for non-genre specific categories. Perhaps i should add a categorization field to series, such that they can get the categorization of "Manga, manwha, doujinshi, novel, artbook"...

Categories might do a lot...hmm...and then the next thing we'll have will be category requests instead of genre requests~

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