It is that they are “muktza” due to being “Shtarei Hedyotos”. (Not really Muktza, but subject to that Issur)
Similar to any paper with ads in it, which is actually worse. Shelas Shalom (such as your novel, assuming that is what one holds) is only a Gezairah for real shtarei hedyotos.
(Which is why there has been a movement to add to them “please do not read on shabbos”, as I have seen a number of times).
The exception for jewish novels is possibly due to them having the halachic status of a sefer, due to their mussar message.
See SA OHC 307:13 for more info.