Between these five novels you can see the transition that science fiction was undergoing at that point. They'd Rather Be Right and Double Star represent the older, pulpier style that was starting to fade away as that early young audience was growing up while The Demolished Man, The Big Time, and A Case of Conscience pushed toward the more literary style that would mark the 1960's. Conscience is particularly notable here since you can almost see the transition in the novel; the background is pure pulp science fiction but the story that Blish chooses to tell with it is much more sophisticated than those space operas.
In the transition of styles I found Double Star, The Big Time, and A Case of Conscience were particularly character driven with different degrees of success. The quality of the narrator in Double Star elevated the weak Prisoner of Zenda plot in Double Star to something special, but I found the narrator in The Big Time to just be grating. A Case of Conscience doesn't let us be so intimately familiar with its protagonist but the exploration of his religious beliefs is the core of that novel.
Of the authors represented so far only Heinlein and Leiber win any future Hugos for novels; Heinlein had a long standing record of most awards for novel until Lois Bujold tied him a few years ago (and as of this year Vernor Vinge is in striking distance as well). With Blish I think that his best work was in the 1950's and his work from this point on isn't very remarkable (yes, including the Star Trek television series adaptations). Bester's novels after the 1950's also weren't of note but his other great novel, The Stars My Destination, would have been a strong contender for a 1957 Hugo if they had given awards for novels that year. As for Clifton and Riley, only Clifton wrote a novel after They'd Rather Be Right, Eight Keys to Eden, and I haven't read it so I won't offer any comment.
You can see the creeping cold war fears start to express themselves in the winners. The first three winners are promising: humanity is becoming something greater in The Demolished Man and They'd Rather Be Right (though I'd debate the improvement on that one) while Double Star features the humanity expanding into space with a kind of manifest destiny. Then things turn around. The Big Time has both sides in the time war as shadowy and remote while the soldiers aren't sure why they go on. A Case of Conscience has a humanity morally weakened by a long running cold war.
1959 was the first time that the entire final ballot was published. The nominees that year were:
- A Case of Conscience by James Blish
- We Have Fed Our Seas by Poul Anderson
- Who? by Algis Budrys
- Have Spacesuit, Will Travel by Robert Heinlein
- Time Killer by Robert Sheckley
To break down my feelings on the books so far to a simple binary scale:
3 - The Demolished Man, Double Star, A Case of Conscience
2 - They'd Rather Be Right, The Big Time