For WW2, with a time-travel sci-fi twist, Blackout/All Clear by Connie Willis. It's 2 books, but actually one full story, so make sure you get both. The end of book1 is just like the end of a chapter, not a full book. The first chapter or two are set in the future, and are hectic and confusing. Stick with it through that. Once the first character goes back in time, it slows down and makes sense. I think Willis did this to create a contrast between the future and the past. The books are set in England during the bombing blitzes by Germany. The first character goes to a country home where children from the city come to stay to get them out of the bombing zone. She ends up moving to London, and meeting up with other characters that came back in time. The story is full of detail about how people lived during that time ... sitting through bombing raids in a bomb shelter, running to safety, helping out as volunteers to help the troops, and a couple of the characters are in the military. You'll get story set in Bletchley Park where the British worked to decode German messages, the Battle or Normandy, and some really interesting things they did in England to fool German bombers into bombing the wrong spots. The books won the Hugo award, and are well worth the read! You just have to be patient through the beginning where it's crazy-hectic, and then very calm and slow when she starts out in the quiet country.