This bit of 1963 sci-fi craziness starts with a cool, overly relatable premise -- a plague of viruses has wiped out 90% of the population of the U.S. (maybe the world, we don't know) and many survivors live in huge, empty underground bunkers built for survivors of a nuclear war.
Now they're empty, but they have stockpiles of food and water to last centuries. Power, water, and other necessities are basically free, if you can find them. And you don't have to work, but some folks do out of boredom.
Cool. Then comes the plot, which is weird and convoluted, and involves many things that might be magic, or illusions, or technology, or something; It's hard to tell. Our hero, creatively named Sam, wanders around without much interest in anything until, whaddaya know, he meets a girl who changes everything.
There's stuff that reminded me a lot of Logan's Run (everyone's a bit promiscuous and clothes seem largely optional) and, as you go along, more and more Wicca. Wiccans are a big feature of the book, and their powers/skills.
We never know exactly what's up with that, other than that they've lived other lives or something, it's never really explained in depth.
Far too many of the obstacles are overcome by powers Sam has that he didn't know he had until right when he needs them (others do similar things, though, presumably, they knew they could). It's hard to hang a plot on that sort of thing, and several times it stretches credulity.
Still, a lot of the book stretches credulity, and that's half the fun. I mean, the FBY (Federal Bureau of Yeast, I kid you not) battles it out with Wiccans, people eat purple fungus, there's a two-brained swimming dog, a lot of rat tides, a blobby monster, an escape though an autoclave, escalator chases, grenade battles, mystic powers, a poor chubby guy who loves building miniature landscapes (St. Clair is pretty rough on the only overweight character in the book, sadly), and a dude who falls in love with a relative (then, fortunately, someone else).
It doesn't make a lot of sense, ultimately, but the post-apocalyptic world St. Clair invented is fascinating and would make a great movie. It does remind one of Omega Man, Logan's Run, Silent Running, and Soylent Green all rolled into one.
And, curiously, this book is listed in the famous "Appendix N" from early editions of the Dungeons and Dragons game, where that game's creator, Gary Gygax, lists books that inspired the game. Though Sign of the Labrys seems much better suited to Gamma World and Metamorphosis Alpha, two post-apocalyptic role-playing games from the 1970s and 80s. James Ward, who created both, surely read Sign of the Labrys first.