A Matter of Oaths - Helen S. Wright
I'm always on the look-out for free science fiction ebooks and this particular one was recommended by Jo Walton, whose writing I also like, over on the Tor books blog.

The basic premise of A Matter of Oaths is that there are two immortal Emperors, with the balance of power between them held by the Guild - a military group operating the majority of space travel in the universe - with binding oaths sworn by both Emperors and Guild members to the Guild. The breaking of an oath is considered serious business, with extreme punishments meted out to those who betray their Emperor or the Guild itself.

Each ship is run by a group of Three, between them taking responsibility for the ship and its crew, with much of the actual work of running the ship undertaken within the Web, a kind of total-immersion internet. We first meet Rallya, a commander, when she's looking to fill a vacant berth on her ship and encounters a man whose former identity has been wiped, which is usually the punishment for Oathbreakers. Somehow, Rafe impresses Rallya enough that she takes him on and subsequently discovers that not only is there much more to him than meets the eye, she's about to find herself in the middle of more trouble than even she can handle.

I very much enjoyed A Matter of Oaths, which is a prime example of the best that the 'space opera' sub-genre can provide - a strong storyline populated by interesting characters, that's not afraid to take the plot into places you might not expect. I understand that the author is planning to write more in this universe, but in the meantime A Matter of Oaths (which is sadly out of print) is available from her website.