CYTEEN; By C.J. Cherryh; Warner; $18.95.

After a Hugo, a Nebula and a long string of best-selling science-fiction paperback successes, C.J. Cherryh has finally made what is being publicized as her hardcover debut.That isn't precisely accurate, because her science fantasy "Angel With the Sword" appeared in hardcover a couple of years ago and a number of her novels have become Science-Fiction Book Club selections.

This is the first hardcover, though, in the series of interwoven novels set in the universe of the award-winning "Downbelow Station," and as such it's an important milestone not just for her but for science fiction in general.

Her greatest work is a series of four long novels - "Downbelow Station," "40,000 in Gehenna," "Merchanters' Luck" and now "Cyteen" - that trace the progress and politics of man's expansion into space. Other pieces of her writing tie into this series, but these four books are the core of an achievment so sweeping that the comparison that comes to mind is with the "Foundation" series of Isaac Asimov - though that comparison is probably not fair.

I say not fair because, in spite of Asimov's near-legendary reputation, Cherryh is in many ways a much better writer. The characters in her major works are fully realized, multi-dimensional people, with all too human faults to match their heroics. Her dialogue, besides serving the obvious function of advancing the plot, sounds exceptionally natural, without the stiffness that marks so much science-fiction writing. And she shares with Asimov and other great SF authors the ability to combine an exciting story with a thoughtful, imaginative theme.

In "Cyteen," though it is also a very political novel, the main theme is the ethics of bio-technology - the creation of artificial human life. The Reseune laboratories on the planet Cyteen turn out "azi," test-tube babies destined to be the workers and soldiers of Union, one of the alliances struggling for supremacy in Cherryh's universe.

The focus of the novel is on the cloned "child" of one of the great leaders of Reseune and Union: Will her carefully controlled genetics and upbringing produce a mirror image of her brilliant mother? And how will the people around her react to this "rebirth" of a woman some honored and many others feared and hated?

The greater issue, though, is artificially produced life itself. The azi may have been grown in artificial wombs, but they're still human beings, set on paths about which they have little to say and lacking most of the rights of naturally born humans.

Some azi of the highest rank are virtually on the same level as their masters, yet they know they truly aren't, and their predicament adds another layer to the questions raised by Cherryh.

Like most fine science-fiction writing, "Cyteen" presents issues that may someday (and not such a distant day) become very real. And it is yet another tribute to Cherryh's skill as a writer that she manages this in one of those stories that you wish would never end.