When the Empire Falls is an early space opera by Christopher G Nuttall.
From the description:
One thousand years ago, Earth was conquered by aliens and forcibly annexed into the Galactic Empire. Humans had spread out across the Empire and have become a vital part of its operations, but now the Empire is collapsing. Economic chaos and political strife have crippled it and its Imperial masters have decided to abandon large sections of the Empire, including Earth. All of a sudden, humanity is independent again, cut loose into an interstellar maelstrom of intrigue and treachery.
Still, no worries, right?
Well, theres the pirate forces that are preparing to swoop on the undefended, newly settled worlds, far too close to Earth for comfort. Theres the sudden unleashing of human ambition as the Imperials withdraw from their peacekeeping role. Theres the economic crash that will put billions out of work. There are the ambitious politicians who see their chance for real power at last. There are the last remnants of the Imperial administration along with their technology, ripe to fall into the wrong hands and theres the unknown alien race that is probing the borders of the Empire, preparing to invade
Admiral Glass, last commander of the Sol Picket, has his orders try to hold as much of the human part of the Empire together as possible. Unfortunately, thats going to be just a little tricky ...
This four-part series can be found for free, here.