What level can I start reading books?

I think you can start with graded readers or Satori reader now. Well If you only know N5 grammar then just grader readers level 1, but If you know up to N4 then you can read up to level 2, and Satori reader should also be fine. Once you’re comfortable with this you should try native material, because imo there’s no waiting to be ready to read. The only way to improve at reading native material, is to read native material. And It will be hard no matter what level you’re at. That being said the recommend level to start reading is 20 I believe, and you should be comfortable reading around 30-40 as long as you start reading at 20.

I say you should start reading Satori reader and graded readers now. Once you’re comfortable with that try some native material and if you want help you should join one of the book clubs, beginner or intermediate .