Blood Sky is the fourth part of The After Series. I highly recommend that you read the first three books: Fallen, Cold Light, and Far Shore. You will get much more out of Blood Sky if you take the time to read through the earlier portions of the story first. Relationships are made, and broken, and sometimes repaired along the way.
Traci L. Slatton does a good job of reminding readers about the most significant things that happened in the previous books. This is not meant as a replacement for reading them, though. It is important that you “live through that” with the characters before jumping into the fourth book of the series. One of the things I love about Slatton’s books is that she makes the reader truly care about what happens to the characters (both major and minor).