The series gets progressively better as you go along. Definitely one of the best series of hard sc-fi books I've ever read.
It deals with issues like alien invasion, the Fermi paradox, galactic communication, the human spirit, betrayal, runaway climate change and technological progress.
The revelation at the end of The Dark Forest (book two) is a staggering insight as to why we appear to be relatively alone in the universe. It's so feasible that you'd think it was slap-in-the-face obvious in retrospect, but I've only ever heard a notion like it once before.