The Wanderer won the Hugo for Best Novel in 1965 but I'd never heard of it - indeed, it stands out in that star-studded list precisely because of its relative obscurity.
Leiber's other, and earlier, Hugo winner, The Big Time (1958), seems to be the better known of his sf works, which is surprising because The Wanderer is the considerably more ambitious novel of the two.
The plot is simple: a whole new planet suddenly appears above the Earth causing death and chaos on a gigantic scale to which it is apparently indifferent. As is so often the case, this simple plot supports a tremendously complicated story.
The Wanderer is a multi-viewpoint novel, a web of about 20 (but don't quote me on that) separate narratives, mostly American but with a smattering of others. They include a group of flying saucer cultists from California (even back in 1964…!), a drunken Welsh poet, the sole survivor of the American moonbase and a lone Atlantic yachtsman, to name but a few.
These are slowly pared down as megadeaths ensue in the wake of the Wanderer's appearance.
It's the sheer variety of narratives that make this book such a fascinating read. Different people encounter and respond to the disaster in unique ways, just as you would expect.
The overall effect is dizzying and thus pretty effective. Taken individually, some of the narratives are rather dated - the Harlem spliff-heads in particular spring to mind, as do the saucer cultists in California whose knowledge of science fiction allows them to correctly guess some of the background behind the Wanderer's arrival - but together they create a glittering profile of a world suddenly thrown into chaos.
Leiber's lively style is not to everyone's taste - some of its hipness is rather faded - but it's seldom dull, in fact coupled with the disjointed nature of the book it evokes very well the confusion and panic that must be felt. The last 50 pages or so are particularly impressive. I obviously don't want to go into too much detail about here, but Leiber's imaginings wouldn't look out of place in some modern space opera, an impressive achievement given the book's age.
His vision of the universe almost matches Olaf Stalpedon's in its scale and apparent pessimism. That this vision is given a single fraction of the space that he spends on the Earthbound struggles of his human protagonists is strangely appropriate and serves to make the final impression of an Earth at once insignificant and oddly blessed all the more poignant.
The Wanderer could perhaps be improved by having more variety in the style instead of just the characters and content, as William Faulkner might have done, because the single prose style does sometimes make the book a bit of an effort to wade through, but you can't help being impressed by Leiber's adventurous spirit; it's disaster nothing like John Wyndham!