This publisher's marketing department is grasping at straws. From the back cover: "The reader may be surprised at how quickly the pages turn." Um, that's certainly a phrase that can be construed in multiple ways, negatively being one of them.
But that's Shreve for you. Her novels are designed to move along, to not incur deep thoughts, and to leave you wondering if you just read chicklit or something more erudite. Because if you look for subtext, it is there, it's just "lighter" subtext than what you would expect in literature.
For instance, the story within a story is written to suggest the power of changing the tale you are writing in whatever way you choose, while the novel's narrative hints at the possibilities of doing this in real life. It's a good setting for a tale of a December wedding (gawd, who let that title through, could it be more boring?) between high-school sweethearts who have lived full lives and then re-met each other at a reunion, plus how all their high-school friends have lived or wish to live their lives. But her writing is just too simplistic to firmly frame the subtext.
I wish Shreve would try her hand at a different genre. Wouldn't it be fascinating to see how she'd handle, say, YA or sci-fi?