At what level one could start reading easy japanese books

For reading, you want to have a bedrock of basic grammar in place. This will allow you to make at least some sense out of what you are reading.

Next is learning vocabulary words. You can focus on learning vocabulary words via WaniKani, or you can also use other resources. You don’t need to learn a lot of vocabulary words before you start reading, but the fewer words you know, the more you’ll have to look up.

In order to understand the grammar and to remember vocabulary, you need to encounter them in a meaningful way. This is where reading books comes in. (As well as other activities, such as conversations with others.)

I often recommend the Absolute Beginner Book Club. It’s a great way to discover simple books. The discussion threads have questions and answers about the grammar, and understanding the material. And there will often be a vocabulary list to help with looking up what the words mean.