Spoilers in this review. 
 This book read like a less coherent Da Vinci Code for art majors, and I am unimpressed with both the fake-novel (if the foreword prepares the reader to experience one of the most fascinating books ever written by an incredibly genius author, the book to follow should probably actually be at least close to fascinating and genius), and the fake-meta-commentary of the fake-novel. Though at first I enjoyed the novelty of the story's main conceit - that the two main characters have to communicate with each other almost solely by writing in the book's margins over the course of a year - there was a point at which I ceased to be convinced by it; it felt contrived and unnecessary. Around the same point or maybe slightly after that I completely stopped caring about the panicky, trivial girl with standard daddy issues and the surly, damaged boy who just needed to find the right girl to plumb his secret sensitive depths. I may have groaned out loud when they both reach their epiphanies about how important love is and what their really real identities are and how they should let go of the past etc etc etc. I felt like the beginning of the novel unintentionally(?) prepares the reader for a much more epic and convoluted mystery than the college-age romance that eventually unfolds. 
 In short: this was a solid concept with really great, but squandered, potential. "House of Leaves" used the ergodic lit approach far more effectively, creating layers within layers within layers of narrative all referring to each other in a way that drew the reader deep into its eerie, monstrous world. It was, like S., essentially a mystery story, but it allowed the reader to discover and interpret the mystery independently. In contrast, "S." felt like it was using all of its two layers as nothing more than a cheap marketing gag, and while its narrative is intended to be a puzzle, the reader merely watches other people solve it instead of participating in the solution. 
 Also, not that this affects my overall enjoyment of the content, but this book is annoying as SH*T to read in the bath with all those freaking inserts.