Best Friends Forever, by Jennifer Weiner.
If time was a dimension, and not a straight line, if you could look down through it like you were looking through water and it could ripple and shift, I was already opening the door. This had all already happened, the way it always did; the way it always would.
Weiner is the author of 7 novels, only one of which (In Her Shoes, which was made into a movie) I have previously read. This novel was excellent. The characters felt real, and the writing is simple but good. The ending was a little contrived but I do like endings where it all "works out", so in my opinion it was fine.
4 out of 5 stars.