Self-proclaimed as a "book-reading community for Japanese language learners," this website is essentially a database of books in Japanese and other written resources that are broken down into levels by difficulty. The site emphasizes getting native content into the hands of learners and relies on the language-learning community to grade resources to help organize content. It employs seven levels into which all texts (excluding textbooks) are categorized based on these community ratings. Resources are also organized by category including, but not limited to, manga, light novels, children's books, graded readers, and more.
The site tries to align its difficulty levels to those of the Japanese Language Proficiency Test (JLPT). In a book search page, you can actually sort the books you are looking for by JLPT level. It also contains specialty categories so you can search for books that are free, books that have won awards, or even those in the WaniKani book club.
All in all, this site is excellent for learners of Japanese looking for some guidance and structure on where to begin on their reading journey. It can be a helpful resource when you're searching for just the right manga or a textbook to match your current level. If you are a more goal-oriented person, it is the perfect site for planning which books you will tackle in the future.