Walk Two Moons has always been one of my favorite books. It's really a book for all ages kind of book, because the girl is 12, so she's not quite a teenager, but she's not quite a kid anymore, which I think just makes the book all that more relatable.
Sal is going on a trip with her grandparents to see her mother (and Sal is going to try and make her come back, though she doesn't tell her grandparents that.) Throughout the whole trip, she's telling her grandparents about one of her friends, whose mother also left, though for different reasons than Sal's mother.
Through these flashbacks we see the parallels to Sal's life, and Sal's troubles. She also tells the reader about her life with her mother, and all the things that she grew up with and the places that she used to go. Inevitably, the story takes place m
ore in the past than it does the present, but it is wonderfully balanced and incredibly written and definitely something that should be read by everyone.
Kurt Vonnegut's Slaughterhouse-Five doesn't have so much "flashbacks" as it does "flash-arounds" because the timeline is all jacked up. The protagonist doesn't really stay in a linear timeline so you don't really know when the "present" is. This is the only Vonnegut book I've read, so I can't say whether I'm a fan of his in general, but I do think this is really excellent writing. I can't imagine trying to keep track of everything while writing like this, and I'm impressed, to say the least. There's just something about being able to reveal the right information at the right time that I don't have the hang of, and it's even more difficult when you've basically thrown your story's timeline into a blender and are reaching in for bits and pieces while it's still on.