A great prequel that really helps settle a few missing pieces from the original three novels. It can be read before or after the series and I didn't feel like I was reading them in the wrong order by reading it after the original three and I don't feel like you will be spoiling the trilogy by reading it before hand either. A worthwhile series to buy and an enjoyable book. 
 It's set in time time after the sun flares that changed the world, but just before and during the attack with the flare virus. It explains how the whole attack with the virus started and the struggle for a group of people trying to figure out what is going on and how to help save everyone from this terrible disease.