Slow Train to Arcturus, by Eric Flint and Dave Freer

In the coolest science fiction, you get to play around with the powers and the limits of the gadgets. You get to undertake incredible missions, and discuss the impact on the travelers as well as those left behind. An finally, for me at least, the real story is about how the author makes people come alive, and observes them being human. And Eric and Dave have done all this, and given us a great swashbucking adventure at 0,3 lights, even throwing in aliens, one of whom has a much worse case of PMS than humans will ever, ever experience.
Somewhere I've got a copy of the ideas Larry Niven came up with for space ship worlds. The form of the ship in Slow Train offers a lot of advantages over some of those ideas, at least in the sense that it doesn't require technology that we just don't have yet. What we have is a series of nickel iron asteroids, heated in a solar mirror and expanded by water flashing into steam. Build the habitats separate them from each other, and populate each one with a group that doesn't want to live on earth any more, for whatever reason. Launch them outbound, and every time you get to a star with a Goldilocks zone, you drop off a bead. Never slow down, so you aren't wasting 70% of your fuel in braking and recovering lost momentum. That's most of the science part, but to emphasize the difficulties that took the colonists out bound in the first -place, an alien race with interstellar capability in decline spots the train, and decides to make rendezvous. Since the bead they land on first contains cannibalistic whack jobs with explosives, things don't go well.
Kretz, the central alien, discovers he has to pass through several more habitats to be rescued, and to effect rescue of his partner. He accumulates a following, as he passes through a zero-tech farming habitat, a Naked Dominatrix habitat, a jungle primitive habitat with a kick, the habitat where people fly (and are engineers) and finally, to a Great Leader habitat,

There is plenty of humor available here, beginning with the fact that the aliens themselves were required to be misfits before they could handle the stresses of a long space voyage. There is a standard language of the alien joke, where Kretz meets Howard, then thinks all humans are called Howard. Differences of sexual dimorphism bring some embarrassing moments, as does priggery and pomposity. Throughout, though, the issue of 'who is the real alien' shifts, each time Kretz and crew enter a new habitat. And when faced with survival, those pre-disposed to learning eventually accept the most divergent.
Those who don't pretty much get blasted between the eyes.
And IF a great story, which this is, HAS to have a message, that's it. But the message never gets in the way of the writing, nor the story.