First of all, this is a review of both Blackout and All Clear, because they are not two different novels, they are one two volume story. All Clear picks up exactly where Blackout left off, as if there was no more than a chapter break. So if you are going to embark on this journey, have both books on hand. I know, I know, its a lot of pages, which is probably why the publisher decided to split the book in half and pretend it was two books, but it just isn't.
That said, the two volume story is well worth reading, I highly recommend it. Its complicated, and full of research and detail heavy, and there are a lot of characters and incidents and locales. Plus its a time travel novel so its twisty, and things don't always happen in order. So its going to ask you to do some intellectual work, no question. But you aren't being asked to work for no reason, the route may be a little challenging but it gets you there, and there is an interesting place that I'm not sure you could get to another way.
Because part of what's going on here is an accretion of detail, of one incident seen from one point of view, and then from another point of view, and then another incident, all of it building one upon another into a wonderful emotionally evocative meditation on courage, and the nature of heroism, and the bravery it takes to simply go on being an ordinarily decent human being in the face of fear.