Sun Of Suns by Karl Schroeder
As of the writing of this review, this book is available online for free from Tor books:
I assume this will not last indefinitely.
This is the first book in a "Virga" series. Although the book ends with some aspects that presumably lead to further books in the series, this could be read as a stand-alone book.
This is an odd one. The premise is an artificial "world" sometime in the future. It's supposed to be a planet-sized sphere filled with atmosphere. There are asteroids moving around inside. It's said to be in the outer part of a star system. It's not clear whether it's too far out to get much light from the star, or whether the sphere doesn't let in the star's light. In any case, the people there depend on small artificial suns - which provide light and heat for less than 100 miles around it. As a result, they have a number of tiny nations each with their own little sun. (Plus there is a somewhat bigger one at the center of the sphere.) Those areas that are not near a small sun are cold and dark, although there is air. Some pirates, outcasts and other adventuresome sorts live out there.
There’s no planet or anything else to provide substantial gravity. Therefore, towns and other structures are often made to be spinning wheels to provide a substitute for gravity. Structures tend to be made out of wood and there isn't much technology. There are a few futuristic items, such as some vehicles that are a cross between a motorcycle and a jet - but these are the exception. Most travel is done by things that are more reminiscent to 19th century (or earlier) sailing ships, although they go through the air and have fuel-burning engines that propel them at a maximum speed of 200 MPH, but usually closer to 20 MPH. Communications are by semaphore flags. Fighting is by sword or bullet-firing guns. And so on.
In this context, the plot is about an impending war between some of these mini-nations. Apparently, a couple of nations that have not been major powers have decided to join together to beat one of the more powerful nations, Slipstream. A hit and run raid is carried out by one of the conspirators against Slipstream's capital in hopes of provoking Slipstream's fleet to go out to retaliate. Meanwhile another of the conspirator nations has been secretly building a fleet that will be able to seize Slipstream. A Slipstream spy discovers the secret fleet building prior to the attack. A few Slipstream ships are diverted from the retaliatory fleet in order to carry out a little odyssey intended to provide them with a new means to stop their enemies.
Their travels take them through the cold and dark regions, to a town hidden in a huge globe of water, battles with pirates, to the fringes of the world and to the central area, on a treasure hunt, etc.
Personally, the 19th century flavor does not agree with my tastes. Not to the extent I would say it was a bad book, but it affected my reaction. Even if the tech was 22nd century, space battles with pirates or straight military SF are not my preferred types of SF. As a result, for my tastes, this will not be such a memorable book, in spite of the unusual artificial world.