The return of Volyova, Khouri and the Nostalgia for Infinity. This book pretty much sets the tone for the rest of the series: graceful (well, OK, not so much in this case) decline and downfall of thriving civilisations. I'm still impressed by Reynolds' ability to pack vast numbers of interesting characters into a hard SF story; I particularly liked Scorpio in this one.
ISBN 0575073845.