Death's End - Cixin Liu
This is an unforgiving book. Did you remember every detail from the last two novels? No - better go back and take some notes!
It is epic sci-fi. Bouncing between times, exploring big ideas, and gently bubbling techno-horror.
But it is uncompromising in how much work it expects you to do. A throwaway line towards the end of the book directly relates to a minor plot-point in the first.
Overall, the three books are a masterwork - but it requires a lot of effort to appreciate.