I love ヒカルの碁 and it’s highly addictive. I recently bought the manga in Japanese, and all my guest family has read the whole story and bought the 20 books I hadn’t bought myself.
I also gave it a try; it’s a little harder than よつばと! but it is still manageable. Plus, the anime is extremely close to the manga, so you can find a little help there (but you need more self control to keep reading since it’s easier to get “the solution”).