I Have Completed the Japanese Duolingo course. Where Do I Go from Here?
I plan on doubling up with Chinese anyway, but with Japanese currently in beta (and I'm not sure if further lessons will be added), I doubt I could pass JLPT N4 with just this alone. Are there any great books or other media that would reinforce the basics and teach at an intermediate-and-beyond level?
If you're dedicated enough, I suggest trying the Japanese to English tree. It doesn't hold back because it's meant for native speakers. It teaches a much larger vocabulary in kanji. The sentences often aren't very natural, though, since it tends to try to convey ideas in ways that native speakers may not, for the purpose of better explaining the English.
For kanji, I recommend James Heisig's "Remembering the Kanji". It teaches the meanings of most kanji (and kanji radicals), and also introduces a method of remembering it (Using the meanings of kanji to form stories + help remember them). However, you should keep in mind that it's not teaching actual words, but rather the individual kanji.
Also, as CereusDelirious mentioned, Tae Kim's "Guide to Japanese" is an amazing source for grammar explanations, when to use what words, etc etc. If I'm not mistaken, some of the earlier explanations he posted also has videos on youtube where he narrates them, but I might be wrong on that/
For dictionaries, I recommend 'imiwa?'. It's an app that allows you to input the radicals of each kanji until you find whichever one you're looking for.
For learning in general, Genki is great too, as others have mentioned. Memrise is good for learning the words (in hiragana- no kanji or katakana until Japanese 3 I think?). Personally, I think Drops is also a really good app for remembering Kanji and the words together, but 1) it doesn't show what context the words are used in (neither does Memrise for the most part), and 2) unless you pay you're limited to a certain amount of time per day.
LingoDeer is also a fantastic resource- it's basically Duolingo, but it teaches Kanji and has a built in flashcard system. It also has Chinese, since you mentioned that in your post. Clozemaster is good for learning in context, and Tinycards has some decent decks for Japanese.
Good luck in your studies!
All great recommendations and I can say they are all very helpful as someone who has used them all.
I do got to add however that the Memorize course does include Kanji right away in the first lesson. I was kind of happy about that. Not only do they include them but, they also have modules that individually explain each kanji they introduce.
It depends what you want to focus on - Japanese from Zero is a book that many people recommend as it's pretty all rounded. Taekim's guide is a bit harder to swallow but the grammar is pretty well explained. If you want to grasp Kanji and remember the many different readings, then I'd seriously recommended signing up to WaniKani. Learn with Oliver is slightly similar to this in terms of the exercises - so really good if that's how you learn. Always lots and lots of videos on YT which are very useful.
The main this is practice! So I'd suggest getting a language app like HelloTalk and posting there everyday - there are native Japanese speakers to correct any mistakes and you can help correct their English in return. There used to be Lang-8 but apparently they've shut down new members? (supposedly they're trying to make their other website -HiNative - more popular) So perhaps try that?
Find a way that works for you. Research suggests that things stick around in your brain a lot longer if you say them out loud ;)
I've heard many people recommend the Genki book series, and I think that should help you (though I've never actually used the book before, and most of the books I use are focused on specific things like grammar and kanji, not a mix of them all). WaniKani is also apparently a good tool to learn kanji with, as it uses a spaced repetition system to make sure you can memorize everything for sure (though it costs money). Anki is also known well by many people to be a good flashcard way of learning Japanese, as it allows you to make your own decks or use other's. Anki is free (unless you own an iOS system, in which for some reason the app costs $25, which is probably not worth it since you could just download it on your computer for all that money less).
As for things I use that probably would be a little less helpful, there's this one workbook called "Complete Japanese Grammar" by Eriko Sato that I've found to be pretty useful since it even has a full answer key (something, for some reason, I haven't been able to find in many workbooks?) and a little dictionary in the back. Speaking of dictionaries, jisho.org is a pretty darn useful Japanese dictionary to use, though I had to get an actual dictionary since electronic use isn't fully allowed at my school.
Anyways, I also like to look around at YouTube for any helpful videos or just plain old websites (but I'll talk about that later) for help on Japanese. A lengthy series I like to watch on there is Japanese From Zero, which has its own book series (that I haven't read yet). It's pretty helpful and the videos explain a good amount of stuff. Japanesepod101.com's YouTube channel is also pretty decent, and apparently a lot of people have learned from it, but I personally don't watch their videos too often.
Now, to make this comment even LONGER, I'll go and mention some websites I like! Tae Kim's Guide to Learning Japanese (www.guidetojapanese.org) is quite helpful for things like grammar (that's what it's known for). Maggie Sensei (maggiesensei.com) is also pretty useful for grammar and etc.
Of course, there's a lot more stuff that I could go on and on about, but that'd make this comment way too long. I hope that this helped you in any way (and sorry if you may already know about these things; I have no idea how much studying you've done outside of Duolingo)!!
2) An Introduction to Japanese Syntax, Grammar and Language (Michiel Kamermans) on Amazon or free (by author) pomax.github.io/nrGrammar/
3) Good grammar supplement a) aforementioned Genki textbooks b) Essential Japanese Samuel E. Martin. Dense but solid work. c) A Dictionary of Basic Japanese Grammar, A Dictionary of Intermediate Japanese Grammar and/or A Dictionary of Advanced Japanese Grammar by Seiichi Makino d) Using Japanese A guide to Contemporary Usage by William McClure (side note (for Chinese) :Using Chinese A guide to Contemporary Usage by Yvonne Li Walls is good [try the 80 page Grammar section starting around page 217])
4) A little more kanji study? a) The Kodansha Kanji Learner’s Course by Andrew Scott Conning (was created more recently than James Heisig's book) b) Kanji ABC A systematic Approach to Japanese Characters by Andreas Foerster
5) Android reference apps which might be useful (if you have an android cell): Jsho Japanese Dictionary, JED Japanese Dictionary and/or Aedict Japanese Dictionary. They should all be offline capable.
Apologies for the length of the post, I tried (and failed) to be brief.
If you want to learn more kanji/vocabulary then I recommend Japanese Kanji Tree. It's an app (free, no ads) that is divided into three sections - recognising, reading, and writing (the reading section includes jukugo and kana-only words, which is great). The app can be kind of dry, but it does what it's supposed to and it's extremely versatile.
As others have already mentioned, Tae Kim's guide is one of the best resources for grammar (it also comes in app form). This one is even more dry than the previous one, but function over form, etc.
Dictionary-wise I would recommend Takoboto, because you can search by radicals, listen to pronunciation, and make study cards.
Lastly, if you want to progress in terms of reading I would recommend tangoristo. It's a reader that has texts (mostly news) of varying levels and you can press on kanji for definitions.
TL;DR: Japanese Kanji Tree for kanji/vocab, Tae Kim's guide for grammar, Takoboto for a dictionary, and Tangoristo for reading.