Five brain-stretching public domain works:
Flatland: a romance of many dimensions, by Edwin A. Abbot (Science fiction or subversive social commentary. Or both.)
Lysistrata, by Aristophanes (Revolutionary politics; the original "make love, not war" story.)
Candide, by Voltaire (The optimist says that we live in the best of all possible worlds. The pessimist fears this is true.)
Call of the Wild, by Jack London (And if you've already read it, but not recently, it's worth re-reading.)
The Chronicles of Clovis, by Saki (Sredni Vashtar, especially.)