A Fire Upon the Deep. Hi-tech meets lo-tech in a story with some rather interesting takes on physics and sentience. Don’t be surprised it you don’t understand anything for a hundred pages or so. It gets easier. A fantastic view of the universe, and amazing aliens. A great journey.
A Deepness in the Sky. Chronologically a prequel to A Fire Upon the Deep but apart from being in the same universe and having one character who appears in both they are completely unrelated. Interstellar travel is slow, and sometimes plans take decades to come to fruition. A mission to a mysterious star finds fascinating aliens who live on a planet with some pretty extreme climate. The mission itself is subverted by tyrants. The novel follows both the aliens and the humans as they both struggle towards the climactic conclusion: Contact!