Preserver is possibly the best of the Morphodite trilogy. It doesn't have the social scope of The Morphodite, nor the singular focus of Transformer, but it's more effective than the other books at capturing one man's curiosity about himself.
Foster takes some shortcuts, but most of the book is a logical, step by step process of discovery. For about 80% of the story, the book essentially functions as a standalone adventure; it's only in the final chapters that prior history really raises its head.
When he gets to it, Foster does a good job of reeling in the Morphodite's convoluted past. Strangely, he introduces a truly ex machina element that, while intriguing, is not a good fit to the story in the length given to it. It might have made a good separate novel, but seems essentially a copout in the context of the trilogy's key concepts. The wrapup of the story is similarly brief. It's a shame, given the book's good qualities.
The series overall is appealing in concept, less successful in execution. In book 1, Foster stumbles a bit in setting out an interesting society, and an idea for altering it. In book 2, he cuts the story short and provides a paste-on ending. This final book is the best written of the set, but again falls short at the end. If you've read books 1 and 2, this book is a not-necessary but enjoyable extension. If you haven't, you can probably figure out what's going on without much trouble, and without substantially altering your enjoyment of the first two, should you read them later.
Should you read them at all? They're decent books with a good idea, and elements are well done. That said, there are plenty of books that are as good at what this trilogy does, and some that are better. If the blurbs intrigue you, go to it. If you're uncertain, it may be better to keep looking.