I second Vladimir's suggestion of Strugatsky brothers' books... Some are very short & simple, yet profoundly interesting. In a similar vein, a Russian translation of Stanislaw Lem's Звёздные дневники Ийона Тихого can be great reading material for intermediate Russian learners.

Also feel free to check out my list of 100 top resources to learn Russian... I've reviewed a few readers, as well as many other learning resources.