You must read Blackout first, ideally read the two books back to back. All Clear is not a complete story it is the continuation of Blackout. Reading the other books in the Oxford Time Travel series first would also be very helpful to understand the world and their rules and limitations of time travel. It is a very long story, but don't be tempted to skip any of it, there's not as much filler as it appears. Everything connects eventually. I still want her to write more set in 2060 instead of only glimpses.