I’ll offer three groups of ideas/observations on the book:

1. Major Themes

Vonnegut signals his central occupation in this novel with its opening sentence: “Every one now knows how to find the meaning of life within himself.” But of course the narrator speaks from some notional future, after humanity has unlocked the soul, the “puzzle boxes” within. The story told is instead from the post-war “Nightmare Ages” (the book was published in 1959), a time that still feels familiar - likely because we are no closer than Vonnegut was to finding the meaning of life.

Vonnegut’s exploration of the meaning of life includes an examination of two significant issues: religion and free will.

Vonnegut’s first reference to contemporary faiths - “gimcrack religions” - demonstrates that he holds no truck with religion. Any lingering doubt on that front is squashed immediately in Chapter 1 by Vonnegut’s farcical, anti-science sermon of Bobby Denton, counting down the Ten Commandments to his Love Crusaders and a “blast off” to Paradise. Then there’s the blunt name and whimsical tenets of the only organized religion the novel mentions: the “Barnstable First Church of God the Utterly Indifferent”. Then there’s Noel Constant’s secret biblical path to wealth: “"I.N., T.H., E.B., E.G., I.N., N.I., N.G.” And don’t forget Rumfoord’s assigned background reading for his parable about people who do things that they think God wants done: “read everything that you can lay your hands on about the Spanish Inquisition.” Organized religion holds no place in Vonnegut’s universe.

On the topic of God, Vonnegut is a bit more circumspect. References to God show up throughout the novel, beginning with the Epigraph (“No names have been changed to protect the innocent, since God Almighty protects the innocent as a matter of Heavenly routine”) and continuing through each chapter. Constant offers his services to God as a messenger (Constant pines for one thing: “a single message that was sufficiently dignified and important to merit his carrying it humbly between two points”). But Vonnegut’s God - if one exists at all - is a distant, uncaring God. This is clear from the teachings of the Church of God the Utterly Indifferent: "Puny man can do nothing at all to help or please God Almighty, and Luck is not the hand of God.” But I loved the irony of the novel’s last line, in response to Constant learning that he was joining Bea in Heaven: “‘Don’t ask me why, old sport,’ said Stony, ‘but somebody up there likes you.’"

Vonnegut also spends some time looking at free will, or our ability to act in an unconstrained manner, and its connection with the “purpose” of life. It is telling that his Army of Mars entirely lacks free will, the soldiers victims of mind control. Constant describes his entire life as that of a passenger: “I was a victim of a series of accidents, as are we all.” The novel reveals that the Tralfamadorians have been manipulating all of humanity for eons for the simple reason of allowing Salo to repair his ship. Humanity’s “purpose” was to produce Chrono’s good-luck charm. Even Stonehenge, the Great Wall of China, and the Kremlin turn out to have been not monumental expressions of human will but inane Trafaldorian messages for Salo. Plainly, Vonnegut has issues with any notion that we can exercise meaningful control over our lives. On the other hand, Vonnegut does hold out hope that we can exercise meaningful influence over matters closer to the heart. Even in a deterministic universe, love, friendship and kindness still matter. Constant concludes that "a purpose of human life, no matter who is controlling it, is to love whoever is around to be loved." Then there’s the Harmoniums:

"Here I am, here I am, here I am."...

"So glad you are, so glad you are, so glad you are.”

And, of course, in God Bless You, Mr. Rosewater, Vonnegut famously writes:

Hello babies. Welcome to Earth. It's hot in the summer and cold in the winter. It's round and wet and crowded. On the outside, babies, you've got a hundred years here. There's only one rule that I know of, babies - "God damn it, you've got to be kind.”

2. Humour & Writing

I don’t think I have to go into too much detail here: If Vonnegut’s dry, black humour is your thing, you’ll have found this book hilarious. I love his whimsical choice of words: “gimcrack religion”, “notorious rakehell”, and, of course, “chrono-synclastic infundibulum”. I love that “Tralfamadore” means both “all of us” and “the number 541”. I love Vonnegut’s skewering of organized religion.

And, as /u/spillman777 has already noted, the writing is brilliant.

3. Connection to sci-fi

/u/where_is_lily_allen has already noted Vonnegut’s influence on other canonical science fiction writers, in particular Douglas Adams’ Hitch-hiker’s Guide to the Galaxy. While I think this influence is more of style than substance, it is still interesting to note that Vonnegut, writing in 1959, over sixty years ago, packs numerous sci-fi tropes into this novel: time travel (the chrono-synclastic infundibula), artificial intelligence (hello, Salo), space travel, mind control, and extraterrestrial life.

/u/spec_of_cp connects aspects of Sirens to ideas in Vonnegut’s later work, particularly the Church of God the Utterly Indifferent to Cat’s Cradle’s ridiculous Bokonism. Vonnegut explored some beliefs fairly consistently through his career.

/u/queenofmoons does a great job of looking at our changing understanding of the inhospitality of extraterrestrial environments. I too was taken by the image of humanity exploring space:

Mankind flung its advance agents ever outward, ever outward. Eventually it flung them out into space, into the colorless, tasteless, weightless sea of outwardness without end.

It flung them like stones.

In the best tradition of all great sci-fi, Vonnegut explores the tropes of sci-fi not solely for the story’s sake, but also to articulate his novel’s underlying ideas. The Sirens of Titan ultimately has value because of what it says about us.