Can't remember the last time I've anticipated a book release as much as the third book in the "Three Body Problem" trilogy, and, I'm happy to say, it did not disappoint. Looking back at the series, book 2 was definitely my favourite of the three; the only book I can think of that blew me away similarly was "Startide Rising". Book 3 started off fairly weakly, but got very intense towards the end. Don't want to say anything spoilery about the actual plot and events, but if you like the "human motes in a very large cosmos" feel of Clarke, Brin and Baxter you should definitely not miss this series.
Also, I can't remember if I've recommended Malka Older's "Infomocracy" on here before, but if you haven't read it, do. Here's a great review by Annalee Newitz. It's one of my favourite sfnal subgenres, exploring ways information technology can help power a renaissance in grassroots democracy and community/tribe building[1], and it goes into more depth than most such stories I've read. Looking forward to the next two books of this one too.
Another "book 3" I've been anticipating was Jo Walton's "Necessity", book 3 in the Just City trilogy. The series had a really intriguing premise; the Greek goddess Athena decided to collect her followers from across the time stream and transport them to pre-eruption Thera, to found a city that put Plato's Republic into practice. Book 3 took the story in a whole new direction (I can't say "unexpected" because it was foreshadowed by the end of book 2), and delivered a nicely satisfying ending to a lot of story arcs, while still leaving the overall sense of an ongoing narrative. I can see her returning to write further stories in the same universe if she wants to - she's left room for it.
[1] and if people have other recommendations I'd love to hear them; thus far the best recent one I've read was Karl Schroder's short story "Degrees of Freedom", in which a group of First Nations people take their country back from a corrupt Canadian government.