Which book to use after Tobira?

From the extract that @Kumirei posted, it does look pretty good. However, are the resources you need available? (E.g. for Aozora, can you get answers to the exercises easily as well? If not, you’re getting a ton of work that you could potentially do with no way of checking if you’re right, which is a waste of time, in my opinion, since that time could be better spent on a textbook that allows more immersion.)

Beyond that… I guess it’s also a question of how fast you want to advance. How much are you willing to study on your own? How much vocabulary do you want to absorb at once? The Aozora passages in that sample are not easy (I had a little trouble with them), but they’re not that hard either (quite a bit can be deduced from context, and the gist of each word is clear even if its precise meaning is not). I think better examples of books that are truly difficult and advanced are these two:

Perhaps I’ve improved a little since I last flipped through this sample, but I was once at a point where even just the 中・上級 textbook gave me roughly three new words a sentence, even now, I think there’s at least one word I feel the need to look up per sentence. These textbooks from the University of Tokyo also seem to contain relatively few exercises compared to the amount of readable text/vocabulary lists, which I feel is a plus for self-learners. The 中・上級 volume was actually used in my country’s high school curriculum in a programme that’s meant to bring students up to… well, OK, I don’t know what the Japanese department aimed for or guaranteed, but the French department (which I was a student in) aimed to get us to C1. My friend, who took the course, said this book should be sufficient for a solid N2 level, with an N1 becoming attainable with a little additional grammatical self-study. I’d say N1 is roughly B2-C1 though, so I’m not sure how to think about it. Then again, my friend’s teachers did provide rather advanced additional materials, so perhaps C1 really was the goal of the course even if the textbook was only roughly good enough for a B2, and my friend did pass the N1 with rather good marks (160+/180, I think). Either way, I think it’s a good book. Whether it’s actually better than Aozora for your purposes is for you to judge. Both look fairly challenging.

I’d like to respectfully disagree. I had a respectable (70+%) pass for my B2 French diploma, which I think safely put me in the ‘upper intermediate’ bracket, but I would have progressed much slower without my guided immersion textbook (essentially immersion + targeted grammatical explanations). The proof: I saw how much more my classmates struggled than I did, even though some of them were at the same level as me when we started our last two years of French language education. I’m now studying in France, and I have a C2 diploma in French with a fairly high score (84% overall; 90% for reading and writing). I’ve also regularly outscored native speakers in French within my stream at university (a science stream, granted). Textbooks may not be as necessary at the upper intermediate level, and it’s true that advanced textbooks are sorely lacking in the Japanese education sphere, but good textbooks are brilliant accelerators. If you don’t have one, or you pick a bad one, you can still reach the advanced level after some time, but having a textbook that really immerses you while proving just enough explanation for you to advance, while aggregating tons of useful advanced structures and vocabulary so you don’t have to seek them out yourself… that makes you tons faster. Many people felt a huge jump when we went from the B1-B2 intermediate level to studying C1 advanced material for French, but thanks to just a few months of extra study with that advanced textbook, I felt almost nothing, notably because it had already fed me all the rest of the advanced grammar and some of the rarer literary words that I needed to be comfortable at higher levels. You don’t need one, but in my opinion, and based on my experience, the fastest route to the top from the upper intermediate level is

  1. Finish an excellent intermediate-to-advanced textbook while doing immersion/other exercises on the side
  2. Pour all the rest of your time into immersion while using a monolingual dictionary as much as possible while reading all the definitions and examples that you have time for

Having someone to help with output practice is of course a welcome addition to all this, but sometimes that option isn’t available. In any case, I think that this is the faster route on the input end, at the very least, even if it won’t necessarily help with output without practice.