This series is absolutely brilliant. If you've tried to learn to read Japanese you know that there are very few books available that are simple enough for a beginning reader to read, but that are not children's books. (And you may also be aware that children's books are not appropriate for serious study because the language in them is so different from adult language and the grammar is too complex.)
These boxed sets of slim volumes look like gorgeously illustrated children's books, but they are written in normal Japanese and are geared towards foreign language learners. The vocabulary and grammar slowly builds as you move up in level. The text is repetitive to aid in remembering and the illustrations are great for aiding understanding. I haven't had to pull out a dictionary yet even when encountering new words because the context and illustrations provide the definitions clearly enough.
And the stories are so engaging! I almost cried at the end of one, others are uplifting. Although the sets are expensive, I feel they are worth every penny. I hope Amazon decides to offer these so that I can add levels 2 and 3 to my wishlist, until then you can find them at White Rabbit Press (use google) or at Kinokuniya in New York.