The Japanese Graded Reader NIHONGO YOMUYOMU BUNKO series (レベル別日本語多読ライブラリー にほんご よむよむ文庫) consists of a huge variety of graded readers, divided into five levels (Level 0-4) in ascending order of difficulty. Each level is then categorized into multiple "volumes," or box sets of graded readers. For example, if you buy Level 0 Volume 1, you'll receive a set of six individual books, each with one story that fits that level. In total, there are 15 volumes and 78 stories available, so you've got loads to choose from.
These books are made by the same company that publishes a lot of Japanese graded reader content, NPO Tadoku Supporters. While their website hosts plenty of free e-books, the Japanese Graded Reader series is something you can actually get your hands on. You can also download the audiobooks if you'd like some listening practice as well.
The topics presented in each box set contain a wild variety, ranging from lighthearted and silly to genuinely heartbreaking.
The Japanese Graded Reader series offers a nice, clean layout without squeezing the print together too closely or being hard to read (besides the fact that they're in Japanese!). The topics presented in each box set contain a wild variety, ranging from lighthearted and silly to genuinely heartbreaking. Each book's illustrations are stylistically unique, and even the higher levels still include pictures. Be aware, though, that the tone or other elements of some stories may strike you as old-fashioned.
At Level 0, much of the content is simply vocabulary, like a picture book for kids, but the difficulty ramps up later on. In fact, when you hit Level 3 or so, you may be able to move on from graded readers entirely. But in any case, if you prefer physical books to e-books and don't mind paying for graded readers, you might find these up your alley.