One thing I would maybe recommend is, rather than Mandarake (which is really cool, but shipping can add up and it’s all about used goods), if you don’t know exactly what you’re looking for, I’d recommend the ebook site Book Walker.
If you’ve got a computer or tablet you don’t mind reading lots of stuff on, it’s an easy way to rack up a lot of books without needing to deal with shipping.
But even if you don’t, it’s still useful since you can click the 試し読み for a short preview of anything, so you can use it to check if you think you might like something beforehand.
For example, here’s what’s been recommended to you so far:
A few recommendations from me:
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機動戦士ガンダム THE ORIGIN and/or 機動戦士ガンダム サンダーボルト, said you like Gundam, so here’s Gundam manga! I think technically there’s probably anime adaptations of both of these, but THE ORIGIN in particular is… already an adaptation of the original anime, so does it really count? If you have any affection for older Gundam, take a look at the art in here, as it’s by the original character designer for the show and it’s stunning.
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ダンジョン飯 - I have absolutely no idea if this lines up with your tastes or skill level, it’s just my favorite and it doesn’t have an anime adaptation. So it came to mind.
Of the three points, “likes mecha,” “~N4/N3,” “avoid manga with anime adaptations,” the last point is maybe going to be the most limiting for recommendations… Especially when combined with the mecha point since that’s probably the kind of genre that would get adaptations quick, if they weren’t already tied with an anime. Would you have any interest in reading the original version of a show you already love? I think that’s how a lot of people make the jump.
How would you feel about manga where the anime doesn’t cover the entire ground of the books?
That could widen the net a bit for stuff like Jujutsu Kaisen…
Otherwise, book clubs here can be a good source of recommendations in general, since a lot of them nominate what to read next. You can pick the skill level you want and look up anything in the nomination list that sounds interesting.
OH also, this kind of search for reading material for learning is pretty much exactly what https://learnnatively.com/ is made for.
Someone on the forum is in the early stages of developing that, so it might not be the most robust tool yet, but you can bug the creator in that thread if you find ways it could be improved.
EDIT to add another rec: Naoki Urasawa might be up your alley? His work is definitely mature and complex, but with still a young enough target audience that it could be manageable, sci-fi tinged, and it seems like it doesn’t get adapted into anime too often.
And for him Mandarake would actually be a great fit (I say with multiple Urasawa sets still in the Mandarake packaging…)! Since it seems like he doesn’t have ebooks of his work (that I’ve found anyway).
- 20th Century Boys (20世紀少年) has a lot of political intrigue and twists and turns. It was adapted into live-action films but not anime.
- Pluto (PLUTO) is about robots, not mecha, but same difference right?? It’s a riff on a famous Astro Boy story but you don’t need to know the original to enjoy it.