Existence is a big book. Not just in page count, though at 500+ pages, it certainly qualifies as a door stopper. But big in ideas, scope, and ambition. It was reportedly a decade or so in the making, and the work shows in the result.
The setup is quite simple. Later in the 21st century, a space garbage man stumbles across an alien artifact. The exact nature of this artifact and its ramifications are impossible to get into without major spoilers. Suffice it to say that this is the springboard that Brin uses to launch into examinations of space exploration, politics, information technology, genetics, and just about every other sort of scientific related issue that you can imagine. Both the amount and scope of the ideas examined in this novel were truly amazing. Almost every page was stuffed with more ideas than fit into an average SF novel.
Brin tackles head-on the current malaise and sense of stumbling into a dystopian future. In fact, this sense of pessimism and failure of ambition are called out to task as our ultimate dangers. His belief is that only a sense of optimism and continued forward progress can solve our current (and future) problems. We must find our strength in diversity and continue forward rather than meekly await some dismal future.