Crystal Rain
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When the book opens, the only peaceful group of humans on the planet are just about to be overrun by primitive Azteca invaders, who practise human sacrifice frequently. The more civilised society attempts to defend itself, but town after town falls to the invaders resulting in massive slaughter and human sacrifice. If human civilisation is to survive on the planet, something dramatic will have to happen. And it does.
If you want to know more, the first 1/3 of the book is online free for downloading chapter by chapter. The chapters are short, and the pace is fairly crisp. Go to the link above for much more information, including background notes by the author for some chapters. Crystal Rain