Existence, David Brin.
Existence is a science-fiction book set in the near future. The seas have risen, migration and inequality are big problems, an ultra-rich trilly ("trillionaire") caste inhabit the upper reaches of society and the interconnectedness of everything and everyone is orders of magnitude greater than today. In addition, privacy doesn't exist anymore, for anyone.
That privacy is no more appears to have been the result of a pact with society after a great disaster (referred to as Awfulday), a mechanism to dampen desire for violence and revolution through transparency. Some of the background in the novel is never fully described and has to be picked up through clues.
It's a long book and, in parts, slow. However, pushing through some of the slower and duller sections (earlier) isn't too hard because chapters are often short and it is so full of interesting and sometimes offbeat technology (including AI, massive crowdsourcing, brain/computer interfacing, genetic engineering, virtual realities and others).
The main part of the book concerns the discovery of an alien artifact in Earth orbit, what it is and how it affects us. In large part, a discussion of Fermi's Paradox :
The Fermi Paradox is the apparent contradiction between the high probability extraterrestrial civilizations' existence and the lack of contact with such civilizations.
If the universe is so big, and the probability so high that it contains sentient life, where is everyone? This is not an uncommon field of interest to the science-fiction author but Brin manages to add his own interesting take. The later parts of the book set in the asteroid belt with discoveries and chatter very pertinent to this question are superb.
Apart from being set in a very uncertain future world racked by a lot of problems and containing a fair amount of apocalyptic background chat, the book is definitely not downbeat or depressing. Brin's a good writer (although some bits should have been excised) and the overall arc is upbeat and positive, even if one has to be somewhat nervous with some of the potential implications.
I enjoyed the book and will definitely add his Uplift books to my queue.
Brin's Existence site seems to be full of material, including sections of the book and discussion.