This one’s something of a go-to
The sparrow is a multi-award winning philsophical “first contact” novel. If Tor is any indication, it’s fairly controversial in the sf community and I’ve even seen it called “torture porn”. It’s also one of those books whose sf status is the subject of some dispute. Nancy Pearl (the basis for the librarian action figure, among other things) claimed in a 2001 Library journal article that it was mistakenly classed as sf, saying it should be classed with general literature because of its philosophical content.
There’s a lot of crossover appeal here, and I recommend it constantly to non-sf fans, but being a “philosophical novel” does not disqualify something from being sf. If that was the case then everything Le Guin has ever written and most of Philip K. Dick’s work would not be sf. I can see putting it with fiction because of the crossover appeal, but claiming that it’s not sf goes a little too far. It was in the general fiction section at the library where I worked when I read it.
(the end of the novel is revealed in the first chapter. The plot is not so much an examination of “what” happened but of “why and how” did this happen)
In 2019 humans find the first proof of extraterrestrial civilization in the form of a musical broadcast. While the UN debates about how to deal with this, the Jesuits send an expedition to the world led by linguist and priest Emilio Sandoz.
Sandoz is the only survivor, and returns broken in spirit and physically maimed.
The novel alternates chapters between the investigation of the aftermath of the expedition and the story of the expedition itself.