The story itself is not for sale, so don't go looking for it on Amazon. Instead, it's available free of charge to anybody who read the book. You can check out Alex's blog post for more details, but I think it's a very cool sort of 'gift' for the fans.
Dragon's Destiny is not a story I can really review without getting deep into spoiler territory for the book before. After all, you need to understand the dark, bittersweet ending there to appreciate what Alex has done here. I will say, however, that he manages to write a convincing, uplifting epilogue that doesn't take away from the sacrifices that led us to this point. It's a complete shift in tone, of course, but that's entirely the point.
As Alex says at the start, Dragon's Destiny is "for those who believe in happily ever after..."