This was a wonderful mystery, with a bit of history thrown in for good measure. If you know nothing about Alaska, it will begin to give you a hint, and, unless you are a top-notch expert, it will likely tell you things you did not know.
The Iditarod is the dog sled race from Anchorage to Nome every March (unless there's not enough snow, as in 2003, in which case they do the best they can!). The race celebrates a famous run of diphtheria serum from Anchorage to Nome to prevent an epidemic. It is a grueling race, on both humans and dogs, and would be much worse if the fictional events in this book ever happened.
In the book, accidents begin to happen to the mushers (that's the human part of the dog sled team). First, one falls off his sled and dies, then one's harness breaks, and so forth. The Alaskan investigators know it has to be one of the mushers or a conspiracy, as opportunities for non-mushers to cause this type of havoc are minimal. Since in each case, even if the results were not fatal the mushers would be out of the race, the investigators focus on the mushers. The race is over 1,150 miles long, most of it in the wilderness that is still Alaska. Stopping the race is unthinkable, but letting it continue may mean more deaths.
Sue Henry does a wonderful job of pulling the reader in and keeping the attention on not only the race for finding the murderer, but the race for the finish line. My only complaint was that I couldn't keep all the mushers straight, because there were so many. But the "leaders" of both races became clear at the end.
Here's a link to the Iditarod website, if you want more information: http://www.iditarod.com/last_race.html