Simple but interesting German literature

Question

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I'm looking for simple but well-known and interesting German literature to improve my reading comprehension as an intermediate learner. It should fulfill the following criteria:

• Well-known: Either classical literature, or contemporary and popular.
• Interesting: Should not be written for young children. Harry Potter passes, Green Eggs and Ham does not.
• Simple: On average, the sentences should not be very long, and most words among the 4000 most common. To quantify, let's say around 70 German Flesch Reading Ease. I took some (too small) random samples and found that Schätzing's Der Schwarm scores 30, while Zweig's Schachnovelle scores 55 and the Harry Potter translation 75.
• „Standard“: Not much deviation from Hochdeutsch, and not much slang.
• Original: Originally written in German. Revisions that make the work conform to neue Rechtschreibung and update archaic words are welcome but not necessary.

Please detail any other properties that make the book useful for learners. One book per answer, please.

Answer 1

Actually, I once had a flatmate from Canada for a year and gave him several books to read, and he found Pippi Langstrumpf and Der kleine Prinz extraordinarily readable, entertaining and just right for him – although they are originally in Swedish and French, respectively, but that doesn't stop the fun they give! I'll include them here just because of this positive experience my flatmate had with them. Their simplicity makes them ideal first books for language learners. If you read Harry Potter fluently without consulting dictionaries or stumbling over the grammar, you are already an advanced reader so they will be too basic for you, even though they are both very entertaining for all ages.

Answer 2

Der Alchimist by Paulo Coelho. It was originally written in Portuguese, but the translation is excellent, I find. It's an easy read and it's a fantastic book (it might just change your life, who knows?).