wickedcestus I wanted to write you a big long post that describes everything I’ve tried, why certain things worked for and why other things didn’t. I wrote maybe a quarter of that, but it’s taking too long, so I’ll post what already have then briefly summarize everything else.
The first thing I want to say is that while Japanese and Chinese share a lot of vocabulary and obviously you know a lot of Chinese characters, if you want to someday be able to actually have conversations in Mandarin (I’m assuming you want to learn Mandarin), then that’s not really all that helpful for the kind of stuff you’d study for the first 6 months or so. At the very least, Japanese and Chinese are completely different in terms of phonetics and grammar. Of course, knowing a bunch of characters might make it easier for you to peruse writing that’s way beyond your level, which might help you get an idea of how the language is actually used more quickly than you would otherwise. But my point is you shouldn’t feel bad if it feels like it’s tough at the start or you’re learning very slowly despite having a headstart via Japanese: knowing Japanese will help you eventually, just not so much at first.
The textbook I started seriously learning Chinese with was Beginning Chinese by John DeFrancis, which I picked because it was the only one I could find whose associated audio tapes were read at a normal speed (i.e. not ridiculously slow) by people who don’t sound like the robotic subway announcement voice. It has way more audio than any other textbook I’ve looked at. The potential downside is that thr main textbook only teaches spoken Chinese (there’s a supplement that teaches written Chinese), though that might be a plus for you, since you already know characters. Once you have some Chinese in your brain, you can start mapping the characters you already know from Japanese to Chinese.
The advantage of this book is it moves very fast. I had tried two other textbooks before this whose names I forget. I remember looking at English translations of the dialogues in the last chapters of those books, and all of the sentences were still composed of single clauses. Grammar in Chinese pretty much entirely takes the form of “sentence patterns” rather than inflections, which make up a much larger part of Japanese grammar. I think Defrancis is nice because in that first Beginning Chinese book, it introduces at least one variant of pretty much every “sentence pattern” that is commonly used in spoken Mandarin. In reality all these sentence patterns are much more flexible than a first reading of DeFrancis (or any textbook) might lead you to believe, and the grammar words that are used can be substituted with others to indicate different tones, or word order can be moved around in ways that, after you hear a bunch of the language, will seem intuitive.
It seems like a lot of textbooks used for college courses in Chinese (e.g. Integrated Chinese) are part of some big machine meant to (1) prepare people to take the HSK, and (2) convince people taking Chinese classes against their will that China is interesting. On top of that they have to deal with reality that a college language course is a few hours a week taken by people with four or five other classes that they probably care about more. So they are designed to be consumed very slowly. That is both an advantage and a disadvantage, depending on how busy you are. Though I will say there were lots of grammar points that seem to me essential for expressing anything of substance that were only introduced towards the end of the second Integrated Chinese book. That indicates to me a textbook series that dripfeeds you grammar assumes you’re not going to try to understand any actual Chinese for your first two years, which I think is a big mistake.
Anyways it’s clear where my biases lie. It’s possible that there’s a newer textbook similar to DeFrancis that focuses soley on spoken Chinese, has tons of audio, and frontloads as much grammar as is reasonable. If that exists, I would suggest that! Particularly because DeFrancis was written in the 60s and the digitised tapes are somewhat low quality.
Other resources:
You know Chinese characters and like Chinese literature. Why not learn classical chinese while you learn modern mandarin? I started with An Introduction to Literary Chinese by Fuller before trying textbooks in Chinese. I believe it has an supplementary index based on Japanese pronunciations, which might be helpful to you. The standard grammar for classical Chinese in English is by Pulleyblank. So you can try those two books (or perhaps pair Pulleyblank with another textbook).
Here's a more rigourous reference grammar for Mandarin that I have used. I can’t really compare it to any other, but it’s useful for, say, when you have some specific point you want examples of.
I don’t know how big a fan you are of Lu Xun, though last year around this time I did a deep dive into him and was amazed how much more I like him than the first time I read Madman’s Diary when I was in high school. His stories and essays are all filled with subtleties and weirdnesses that I don’t think I was prepared for back then. Anyways, there is this book that presents his poetry along with word by word literal translations (along with a bunch of analysis). It might be fun to read and has the potential to actually teach you a little Chinese.
That’s all for now. There are a lot more resources, so I might write another post that goes over them with less commentary from me.