This is the first book by Alastair Reynolds I've read, but I'm half-way through Chasm City so I've got that to compare it against. Century Rain, as an earlier reviewer mentioned (hardback edition), does seem more upbeat, or light-hearted (perhaps), than CC - which is quite a grisly affair (but undoubtly with a lot of quality and detail).
To cut to the chase the book centres around an alternate 1959 Paris discovered at the end of an ancient wormhole/portal. There's two main chracters, Floyd - a part-time private detective and part-time musician from the alternate Earth and Auger - an archeologist studying Paris on our own Earth (now covered in ice and suffering from the remnants of a nano-technological virus) a couple of hundred years in the future. Needless to say their two worlds collide as expeditions are sent through the portal to learn about the alternate world and to find out why an agent, sent through the portal earlier, has beem mysteriously murdered. It's gripping stuff with some very good action (not just of the fire-arm kind) and not without a fair dose of classic film-noir paranoia thrown in too.
The quality isn't 100% though, there some bits that near the end that seemed a bit silly, Reynolds seems to "over tell" things at a few points "Floyd stood up, walked to the door and opened it, then walked down the stairs" - that's not a quote but it's the general idea. These flaws are however few and far between. I genuinly felt some of the wonder that these characters from the future must have felt as they walked the streets 200 years in the past for the first time, and there's some vivid surreal imagery on offer.