From the University of Texas, Grimm Grammar uses classic Grimm Fairy Tales updated for the 21st century. From the about page of the site:
The primary audience for Grimm Grammar is the beginning language learner. Each part of speech – e.g., adjective, adverb, noun, verb – is introduced by a sectional overview, and then further explained in relevant, thoroughly cross-referenced sub-topics: e.g., articles, the past tense of regular verbs, etc.
Grammar descriptions in English/German and self-correcting exercises promote the understanding of grammar concepts through plausible communicative contexts – what someone might actually say in real life – using predominantly discourse-length language samples.