J.J. Abrams is a director, writer and producer of both TV ('Lost') and film (the first of the upcoming 'Star Wars' trilogy, for instance).
That may explain why 'S' is more of a very atypical experiment than a classic narrative.
The slipover cover contains an imitation of a 1950s library-book, 'Ship of Theseus' by 'V. M. Straka.
The plot of this 'Theseus' is then
a) commented by the footnotes made by a Straka-specialist (think Alfred Appel commenting on Vladimir Nabokov), and
b) doubled by numerous notes scribbled in the margins by two other people, corresponding with each other through this particular copy of 'Theseus'.
That equals four narrative voices, in all.
The hand-written notes correspond to five different time periods, each represented in a different colour of ink (colorblind readers abstain from this one) but all liable to appear on the same page. (Obviously, no explanation is provided. That would spoil the fun, I guess?)
Add to that loose papers folded between the pages; postcards, letters, telegrams and paper clippings, all relevant to the two penpals' hunt for the identity of the (fictive!) author, Straka.
They crack codes in the novel and footnotes, they rival with another literary researcher and are increasingly persecuted by threatening strangers.
The plot in 'Theseus' parallels the plot panning out in the margins.
= Complicated only begins to describe it!
It is a tribute to Abrams's genius that it still reads fairly well, and is entertaining!
For a book about literature, the style is exceedingly straightforward (Abrams is a writer; not a novelist. Apparently, neither is Horst.) which is a pity.
It is also a bit frustrating that a lot of loose ends are left hanging in the end, but then after 450 pages of keeping two plots, four narrators and seven (in all!) time spans in mind simultaneously, I was also rather thankful to get to move on to something else.