Virga: Sun of Suns

Karl Schroder's book Sun of Suns is set inside Virga, a mini Dyson sphere about 5000 miles across. There is a small artificial sun in the center, around which everything else drifts in slow orbits: lesser suns, spinning cities, spherical forests, icebergs, rocks, and immense clouds. "Winter" isn't a time, it's a location--anywhere that is too far from a sun, or shadowed by cities or other permanent features, and therefore doesn't get enough sunlight. Ships through immense clouds.

Hayden Griffen's mother attempts to start a sun for the city-state Aerie, but the project is attacked by Slipstream forces, a city whose orbit has brought it close to Aerie. Hayden sets out to seek revenge on the man responsible for the attack. He meets his enemy's wife, and a youngster serving in the Slipstream navy, and a woman from outside the world. They discover that an enemy nation is getting closer, and it has dreadnoughts; in order to defeat this enemy, they set out to find a legendary pirate treasure.

The story is interesting, the characters are realistic rather than cardboard, but what's really fascinating is the setting, and the worldbuilding behind it. Definitely buy.