I haven't read any of Rowell's other books, so I don't know if it's common for her to play with time the way she does here.
In this instance, Rowell places the book in a particular time period but through the device noted in the title, adds in the ability to talk to others (or one particular other) in another time period. I'd be surprised if the majority of folks reading this book weren't immediately taken by the idea of being able to contact someone in your past (or future, I suppose). Who would you want to speak to? I'd definitely talk to my Dad when he was still at the beginning of his career (70s & 80s).
At heart, the book is about family, and both families described are definitely "modern families". Rowell has a bit of a pop/hip style, so these descriptions are fun, funny and engaging, and only feel slightly unrealistic (the main character's mom's family has one too many elements of weird). In fact, Rowell's descriptions are by far the best part of this book, from the drawing of a cartoon squirrel, to a child's voicemail to her mom, to what it's like to sit next to an ultra-tense person on an airplane. For that reason alone, I'd read her other novels.