Sakura Tadoku Lab is an online library of graded readers that offers books at eight different levels of difficulty. What this means is that with the added granularity, it's easier to find books suited more precisely to your level.
Furthermore, and perhaps most critically, the website offers a vocabulary test to help assess how many words you know and what level you're at on their scale. In their graded readers, they aim to use vocabulary that people at your level would know about 95% of, just to ensure that it's easy enough for you to read smoothly. This is an interesting and useful approach for all learners of Japanese, regardless of your interest in graded readers specifically. The test gives you a series of words and asks you to pick a synonym for each from a handful of options, all in Japanese.
In their graded readers, they aim to use vocabulary that people at your level would know about 95% of, just to ensure that it's easy enough for you to read smoothly.
On the index page, all the books are listed with their title, cover, and level, as well as the number of characters each work has. The lengths of the works range from around 1,700 characters all the way up to just over 40,000 characters, so there are stories available regardless of how much time you want to spend.
Clicking on a book takes you to the book's page. Here, you can see some info about the book like the title, author, and description, as well as a rating for some books. Below are links to download the book as an ePub or Mobi file for your e-book reader, or to view the story as HTML. This loads a webpage which scrolls from right to left as you read. Like the NPO Tadoku Supporters library, you can't toggle furigana on and off here. But since the stories are rendered in your browser, it's easy to increase the size and even to copy and paste if you want to make notes or use a dictionary plug-in like Yomichan. Finally, some books include a questionnaire at the end, where you can test your comprehension and give feedback on the books. This data gets saved to your profile, similar to your vocabulary test results.
The stories are also rather varied. I read a surprisingly complex story about frogs in a pond, and a retelling of Natsume Sōseki's Botchan. I found it a little unusual to scroll left to view the stories, but quickly adjusted to this.