This isn't a linear book by any means, and the characters go through iterations of the same thoughts and actions over and over but like Bolero it builds to the meat of the plot. Blackout and All Clear are really one book so the climax of book 1 is a bit of a cliffhanger.

It is also important to pay attention to what time each chapter takes place in, or you won't realize the significance of other events.

That said the world building (the Oxford of present day in the story, time travel mechanics etc) are inspiring, and the history lesson woven in the day to days lives is fabulous. The life of shopgirls and ambulance drivers and evacuated children during World War 2 is sketched in mesmerizing detail.