Book 2 of Sales's Apollo Quartet.
In another parallel universe, the space programme kept going. Via the chance discovery of an alien artefact on Mars, mankind is given the key to interstellar travel. A colony is set up on an exo-planet named Earth Two.
Elliott is a veteran spaceman, the only human ever to have gone to Mars and to have seen the alien disc in place. He is selected, via a labyrinth of government secrecy, to join a mission to Earth Two when the colony has a serious problem.
We are still in the same world as the first book, Adrift in the Sea of Rains (see my review at http://chaotopia-dave.blogspot.co.uk/2013/01/review-of-adrift-on-sea-of-rains-by-ian.html ), and indeed still in the same time-frame. But a secret has been kept: only one man has ever seen the only known evidence of alien life.
From the opening lines, Sales sets up the protagonist's driving motivations. He is leaving a loving wife for... what?:
'This time, when he returns home he knows she will have left him for good.'
This is a man to whom space exploration is the be-all of his life. And maybe the end-all too:
'There is nowhere else he would rather be.
'He has missed this-the freedom of freefall, the sense of purpose that comes from following mission checklists, the purpose that comes from following mission checklists, the constant marvel of human engineering, the desperate desire to find a place for humanity in an implacable and indifferent universe.'
The Eye of the title is of course humankind; but is it humankind alone? That is what Elliott would know.
The narrative runs a present-time journey to Earth Two, in parallel with flashbacks to Elliott's original Mars mission. The alien-physics-derived Serpo engine is run from a Rock, a near-Earth asteroid towed to the Lagrange point between Earth and Moon. The world on that Rock is what Sales really excels at, the descriptions of humans functioning in deep space. I don't think I've read any other fiction that comes near in this respect.
The style generated by the mix of effective space exploration with what reads like 1970s tech is almost a 70s version of Steampunk: these are people doing amazing things with old-fashioned, clunky technology.
This technological mix is then juxtaposed with the alien tech. In the first book we had the Bell, weird salvaged Nazi technology. In this one, we have the Serpo engine. This is perhaps less outré than the parallel-universe-spanning Bell, but the story is by no means less exciting. I enjoyed Adrift..., but I prefer this volume.
Small complaints department: There's something odd about the font sizing in the Kindle edition - the characters are at least two sizes smaller than those on 'Adrift on the Sea of Rains', and every other Kindle file I have.
(There's also a typo - 'vaccum'; I know, the process of proofreading self-published books never ends.)
This book really works. Buy it for the same reasons I recommended Adrift..., only more so.