I recommend “時をかける少女” as a first book for beginners

(Toki wo kakeru shoujo)

You might have heard of it, it’s pretty famous. It’s the first book I’ve ever read in japanese and I can tell you it doesn’t get easier than that as far as native material goes. It’s also pretty short. There might be 5~6 pages in the middle where some scientific stuff is explained where it can get a bit harder but you can skip them or machine-translate if it’s too much and get over them fast if you don’t want to force yourself too much. Please try it out if you’ve never tried to read a book in japanese before. I was 3 months into my studies and I finished it pretty fast because the story is also very interesting to be honest. 🙂