I just finished James Blish's A Case of Conscience (1958), which I picked up in a used bookstore today based on its reputation as a classic piece of SF. After reading it, I can definitely see why the novel has achieved its status: it's a well-written piece of SF that's more concerned with moral and philosophical problems--the case of conscience of its title--than with gadgets and technology (though those get their play as well). The second half of the novel, set on an Earth that exists almost entirely underground in “shelters,” is reminiscent of Asimov's The Caves of Steel (though more pessimistic) and the overall tone and feel (along with the personality of the main character, Father Ruiz-Sanchez, SJ) recalls (or, more accurately in terms of chronology, points to) Silverberg's novels of the 1960s and 1970s, particularly Tower of Glass and The Masks of Time.
While Ruiz-Sanchez and the other major characters--the scientists Liu Meid and Michelis and the alien Egtverchi--are not quite as memorable as Lije Bailey and R. Daneel Olivaw, they are fairly compelling. If at times Blish uses them more as mouthpieces for philosophical and ethical positions than as actual human beings, it is justified given the fascinating nature of what they discuss: the nature of matter, the question of evil, and the existence of God and the Devil. Ruiz-Sanchez in particular is a masterful portrait of grief and doubt, a man confronted by a startling revelation that both undermines and confirms his deepest belief.
There are some flaws with the novel--certain plot threads are not satisfactorily resolved, and several major developments are simply a little too convenient (there is no adequate explanation for the destruction of the Message Tree, for example, other than to quell obvious objections to the novel's resolution: “Why didn't they just call them?”)--but they generally do not detract from the experience of reading the novel. Indeed, given that the novel was first published over fifty years ago, it has aged surprisingly well. Certainly it is less cringe-worthy than some of Heinlein's efforts from the same period. It is a great example of SF that is more sociological than technological, and really pushes the limits of what the genre can do without resorting to the (occasionally gimmicky) shock tactics of some of the New Wave SF writers of the 1960s and 1970s.
One interesting point: I was surprised to find a preface from the author defending himself against criticism for suggesting that in the almost century difference between the publication of the novel and its setting (2050) there would be certain changes in Catholic doctrine. Just last week I would have dismissed such a preface as unnecessary and distracting, but after hearing the theology students in the German class I am taking go at it over (what seem to me at any rate) mundane and rather obvious problems (such as whether or not it is acceptable to use gluten-free communion wafers), I can see why Blish felt the need to make clear his reasoning behind the proposed doctrinal changes.
Anyway, I recommend A Case of Conscience to any SF fan (and to all readers, really). Any SF novel that can make its central plot concern the consequences of an heretical opinion (Manichӕanism) by a Jesuit priest regarding whether or not to open a planet for settlement by humans is worth a look. The political and social commentary the novel makes and its treatment of the problem of evil are also fascinating (indeed, I would like to say more about this when I've had time to digest the novel). This novel proved to be well worth the one dollar it cost me (ah, cheap old SF paperbacks. Is there anything better?).