Everyone always asks about music or movies, which is a shame...
You're going to the Mars colony: freight is incredibly limited, as is electrical power once you get there, thus you get one--one!--physical book to sustain your leisure indefinitely. (And don't say any kind of survival manual, they've already got those. I'd also say "don't say the Bible," but that's not likely an issue with this crowd.
)
What one book do you bring?