@Jinyoung After the Genki I and II we moved on to the tobira (上級へのとびら) it is (supposedly) perfect for those who have finished Genki I and II as it is bulit up from there. But if you do buy it don’t be surprised, in this one almost everythnig is written in Japanese, there are only a few english sentences to explain grammar parts. I don’t know any other books unfortunately.
Esperanto Radio https://esperanto-radio.com/
Another resource is Pera Pera Penguin. It was a weekly column in the Daily Yomiuru newspaper until 2011. This website has archived the lessons.
Is it difficult for you to learn Japanese without having much knowledge in the writing system?
I understand most of the system now, but I would say learning the language itself was fairly simple with romaji. The grammar and syntax is a bit different from any other languages I’ve studied, but not terribly difficult. I have the most difficulty with reading Kanji, I’m still learning that particular system. I still have a lot of vocabulary and other things to learn, so each day is a new adventure in nihongo.
Learning hiragana and katakana will help with pronunciation. It also can help you determine how sentences are formed. Nouns are kanji, adjective and verbs are a mix of kanji and hiragana. Prepositions like “to”, “of”, or “at” also use hiragana.
I agree with the others, learning hiragana and katakana are musts, and it’s not that hard to do.
Concerning kanji, it depends what you want to do. You don’t have to learn them right at the begining.