Not sure whether this will "keep the brain working", but it definitely tied some knots in mine: The Space Trilogy [1], by C.S. Lewis, blends sci-fi with spirituality and classic mythology. The books should probably be read in sequence, as they follow the adventures of the same main character, but Lewis's almost fractal writing style in wildly trippy sections in the first two (which bear hallmarks from the New Testament's Revelations), exhausted me. The last book in the trilogy is worth reading in isolation; it deals with problematic uses of science (eugenics etc), government capture, media manipulation/propaganda, and also human psychology/power relationships etc. It also has its mystic/mythological stuff (Merlin of Arthurian legend is a character in the book), but what gave me cold chills was the creeping dystopian takeover of societal norms that certainly finds echoes in our modern world.