[JSWolf]

I'd like to nominate Existence by David Brin.

Billions of planets may be ripe for life, even intelligence. So where is Everybody? Do civilizations make the same fatal mistakes, over and over? Might we be the first to cross the mine-field, evading every trap to learn the secret of Existence?

Astronaut Gerald Livingstone grabs a crystal lump of floating space debris. Is it an alien artifact, sent across the vast, interstellar gulf, bearing a message from far civilizations?

"Join us!" What does the enticing invitation mean? To enroll in a great federation of free races?

Only, what of rumors that this starry messenger may not be the first? Have other crystals fallen, across 9,000 years? Some offering welcome... and others... a warning?