In the future, humanity has spread outward, colonizing all the way to the asteroid belt, and mutations have caused a social divide between those who can survive only in freefall, Fallers, and those who need exposure to gravity to survive, Grounders. There are also Floaters, people who have one of each gene, and can survive in freefall or gravity fine.

At least, that's what everyone has thought. Now, a new scientific discovery has revealed that the simple genetic profile doesn't completely cover it- some children who have been genotyped as Grounders have been proven to be perfectly comfortable in freefall from puberty on- or even from birth.

The lines are blurring, and in an Earth society that believes that Fallers and Floaters are freaks at best, monsters at worst, this is not a welcome discovery. The terrorist group MAMA, Mothers Againt Mutant Accession, is gaining in power and boldness, with deadlier and deadlier attacks...

...and the hot-shot pilot Skyrider is the last to find out about it. She's been taking some time off and living in peaceful isolation on her new rock- with her new lover, Ian Spencer, con man and computer wizard. Isolation and content ignorance- that is, until an infant shows up in her airlock, without tripping any of her proximity alarms- and a group of unmarked ships show up immediately after, gunning directly for the airlock.

The Skyrider has no idea what to do with the infant, but Ian refuses to give it away to an orphanage, hinting at such a past of his own. When Ian takes off after an argument, the infant proves to have the Floater Factor, and some old friends in dire straits come asking for shelter from MAMA, the Skyrider leaves the infant in their care goes on a journey of her own to find some answers.

This is, unfortunately, the last book published in the Skyrider series- but it goes out well. This book is a return to the Skyrider's first-person POV, and e get to see more of the society of the terraformed Mars and a future San Francisco, some older characters return, and some new significant minor characters are introduced.

I wish there had been more, but according to an interview with the author*, the sales on the series weren't good enough. However, unlike some series cut off too soon, the plot threads of this book as well as several that have carried through the series are resolved in a satisfying way. I would highly recommend the series if you are looking for a fast-paced sci-fi series with a tough and likable female protagonist.

* http://www.sff.net/people/jbailey/melisa.htp - note that the story "I Have a Winter Reason" mentioned in the interview, which features the first appearance of the Skyrider, is available in the hardcover anthology Isaac Asimov's Aliens and Outworlders, ISBN 0-385-27912-4. It's an alternate but very similar version to the opening of the first book, Skirmish, covering Melacha's return to Earth and her reunion with Michael, and probably not worth tracking down unless you are a completist. ( )