I can say without hesitation that this is one of the best hard sci-fi series that I have ever read. The concepts are breathtakingly ambitious - the birth of two new species of humans burst over 1,000 years - with wonderfully complex and realistic characters. I have read each of these novels separately several times, with great enjoyment, but this is the first time I have read them in sequence. The quartet is enthralling and, as with the best sci-fi, believable because scientifically plausible.
The first novel is perhaps my favorite. Doro, a seemingly immortal vampire-like mutant who is attempting to breed a race like himself, senses another powerful mutant (Anyanwu). Her follows her "scent" and compels her to accompany him, for breeding purposes. The result is a battle of wills like none that I have ever encountered in fiction: Doro is a cold and implacable killer, but Anyanwu is a healer that respects life. Over the course of over 300 years, they fight, through the lives of Doro's people, her escape and recapture, to a compromise. It is as exquisite as it is bizarre, full of historical imagery and unusual concepts.
In the second novel, Doro in a sense achieves his goal, but the result is not at all what he expected. The surviving mutants exhibit a range of powerful abilities, growing from the destructive side effects of Doro's many semi-failed experiments. There is a new battle of wills, as a new human species emerges. There is also the emergence of a new kind of social organization, a dependency between the two human species. Again, fascinating ideas and characters that evolve with great realism in fantastic situations. It takes place more of less in the present.
Taking place somewhere in the near future, the third novel introduces something unexpected, from space. Once again, a new species of symbionts is born, radically at odds with the rest of humanity. While I felt that this novel was far weaker, it is crucial to the series and full of surprises.
The final novel pulls it all together. Hundreds of years in the future, the old societal order of man has entirely crumbled, much of it literally into dust. As new abilities enabled some to gain socio-political power, the old mechanized culture has essentially vanished. There are three species of human: two are integrated and mutually dependent, the third represents an almost alien enemies, which are dangerously contagious and super-humanly agile. They exist is an unstable equilibrium. The locus of the plot is a struggle for power in one of the groups, whose social organization and super-abilities are slowly revealed. A new leader must take over, but the competitors - full brothers - could not be more different: one is without question the more powerful, the other more subtle and with a different mix of abilities. The climax, during an extremely dangerous trek with war brought by the other species, is wonderfully frightening. Characters are at the heart of this novel, though human potential is also a theme. It is truly wonderful and fascinating.
Warmly recommended. Re-reading these together is a rare treat, in which the reader is transported to an unimaginable future and danger and possibility. It is unlike anything I have ever read.