A Deepness in the Sky

I don't want to go into much length here, but, long story short, this is by far the best, most fully-realized first contact story I've ever read. The aliens are fully realized, with their own, entirely appropriate culture and ways, they aren't just odd for the sake of being odd, while still being interesting, and the massive gulf of interspecies communication is dealt with beautifully

It's a darker view of singularity fiction, what I might call S-curve fiction if there were more examples of it, where the physical laws governing the universe ultimately bring all useful progress to a crawl. It'd be an awesome take on the genre if it hadn't essentially invented it.