I'm learning German and I want to challenge myself a bit.

Kafka has a pretty basic vocabulary but his syntax is sometimes complex and may seem off to a learner. Depends on your progress so far.
Anything "old" like Schiller or Goethe is probably not a good idea as German has changed quite a bit.
Dürrenmatt is easy, fun and still has a bit of substance. Try Der Besuch der alten Dame and Die Physiker. Both are technically plays but are very smooth reads.
Hesse is not too complex in his vocab or syntax, give him a go if you want. Siddartha is a bit more complex if I remember correctly as he was imitating an older style. Go for Steppenwolf instead. Maybe Unterm Rad.
If you want a really fun read go for Thomas Brussigs Am kürzeren Ende der Sonnenallee.

>>9378470
try
Verteidigung der Missionarsstellung by Wolf Haas

>>9378470
Try reading some folk-tale collections. They are repetitive, short, and simple stories. The repetition of the same pattern ("how big x you have") are useful for teaching sentence structures.

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Easy and an absolute must-read for every aspiring patrician.

Hesse is a must, also Böll and Heine an
>>9379362
basically this

>>9379381
>>9379418
Obvious bait. Roche ins Gas

Stefan Zweig - Die Schachnovelle

>>9379699
this