This was not, as near as I can tell, a book translated to a movie. It was a story imagined and created as a film. The script uses several very clever devices.
First off, if you're not familiar with the story, Summer is a woman's name, and the entire movie is the story of 500 days of her relationship with a guy named Tom who wanted to be an architect but settled for writing greeting cards. I am not usually fond of high-overhead stories where the reader/viewer has to keep track of lots of info, but this movie made it fun. The number of the day of the relationship flashes on the screen— , , , etc. I'm making up that sequence, but the days are not shown sequentially and we jump back and forth in time, uncovering clues about what happens as we go. Summer cuts her hair toward the beginning and then lets it grow, which is another visual reminder of where we are in the story.
The story is also shown completely from Tom's point of view. We meet his friends, his hilarious and precocious little sister, and his boss, and see him at his job. For once it's the guy who wants a long term relationship; Summer says she doesn't want to be anyone's girlfriend. We actually see some scenes the way Tom feels rather than as reality. Tom's dance sequence their first "morning after" is utterly charming. We also see Tom's sketches of the LA landscape take over the whole screen. At a critical juncture of the storyline, we see his anticipated events on half the screen and the real events on the other half.
So, if you haven't seen it already, rent, buy, or download this movie! The cast is fantastic, and the story telling is truly original.
If you have seen it, you can weigh in on how you felt about Summer. A lot of people seem to think she was a bitch, but I don't agree. I do think the writers saddled her with one thoughtless action because it made a better story (they could have explained it a little better).
And the little sister is a stitch.