Roseanna by Maj Sjöwall and Per Wahlöö: One of the few works that was revolutionary in a genre that I've found to still hold up well. Additionally, it has gained some interest as a sort of "time capsule", since it covers so many little details of Martin Beck's daily life and work, so many things that have been changed by technological or social shifts in the past half-century or so. I tend to find murder mysteries rather depressing, so I'm not going to be reading further into the series at this time, but it really is an excellent book.
Requiem Infernal by Peter Fehervari: I was astounded after I finished this to find that it was classified as just a regular 40K novel, not part of the Warhammer Horror line. It certainly deserves classification as "horror" far more than most of what GW has actually put out under that imprint. Really drives home that 40K is, fundamentally, a horror setting.
Middlegame by Shaenon McGuire: Amazing book. I think I actually need to re-read it a couple of times to really get it. It's a pretty complex structure, and I kept having to flip back and forth to check things that had happened before, and how the current version differed from previous timelines.
Network Effect by Martha Wells: A full-length Murderbot novel! The one thing I think I can say about the brilliance of this book without spoilers is that adding a teenage girl was one of the best possible choices to make for the series. I mean, Murderbot's primary weakness has always been strong emotions, and teen girls* are like 80-90% composed of strong emotions. It creates some truly wonderful conflicts for our favourite SecUnit.
*Yes, teen boys are too. But they tend to express it in ways that would be easier for Murderbot to deal with.