j-novel club, it's legally licensed to sell the works. You can either subscribe or purchase the pre-pub novels. Not entirely sure if subscribing allows you to read finished novels, tho 🤔 I think it does, but I can't confirm (never subscribed).
There is a time window (I think 3 months) where published chapters are still readable for premium members, after that only when buying the volume or if it goes into the catch-up rotation.
Green = Available to read for Subscription Members
Note: The first chapter in any volume is always free to read, do that before buying the entire book.