In which humans and alien coexist with varying degrees of success twenty years after the partial conquest of earth during World War II, upon the arrival of the alien race's second fleet of ships.
Harry Turtledove is unequivocally the master of the alternate history fiction. His settings are their own special kind of nerdery where time travel, alien invasions or random events change history. So in Guns of the South, for example, the confederate army is supplied with AK-47s by time travelers. His books are an interesting mixture of real historical figures and events and fiction in ways that make (alternate) history come alive.
Second Contact is the first book in his second series in which aliens (The Race) invade earth during WWII. In the first series (the Worldwar Tetralogy which I highly recommend) earthlings are carrying on killing each other in the 1940s when an alien colonization fleet arrives. This book is set in the 1960s and has some of the same characters as the first series and some new ones.
Turtledove writes with lots of alternating third person, limited perspectives which is, no question, my favorite writing style (think Song of Ice and Fire or Wheel of Time if that didn't make sense). I totally go for this style! It's a very appealing narration style to me, especially with complex plotlines in which events are described through different perspectives. I think some people dislike this style, and Turtledove's tendency to have LOTS of characters, but I approve.
The characters in Second Contact span the globe and include Russian leaders, alien colonists, alien defectors, U.S. military personnel, Jewish doctors, alien experimental subjects, Chinese ladies, and German astronauts. It's really all over the place in an interesting tapestry of plotlines that weave together or diverge frequently.
In the Worldwar series events move really slowly. There are four books over which plotlines take shape and plans hatch. For whatever reason Second Contact moves much more quickly, focusing on a much narrower segments of events having to do mainly with ginger smuggling (a popular and destructive drug among the aliens!) and the major human powers' space programs. It was definitely more focused than the previous series, but I don't know if that's good or bad because I really enjoyed the variability and diversity of plotlines in the Worldwar series. People who find a bunch of perspectives and converging stories tiresome might prefer this book to those, but if you feel that way Turtledove is probably not going to be your thing.
In summary, read this book if you enjoy science fiction, but only after you've read the Worldwar series. Start with Worldwar: In the Balance. I can't wait to read the next book in the Colonization series and then maybe some more Turtledove books. I initially disparaged the cheesiness of his settings, but his style really grew on me and I am a big fan now!