I don't think you'll know for sure what's going on until the very end, at which point you'll kick yourself. And that is the point at which you will realise why this book was never made into a movie - you are reading this book as a modern reader THROUGH the eyes of someone of the times in which the novel is set.
A modern movie viewer would see right through it, but reading as though one were part of the story, it's hard to discern what's going on. I found myself very drawn to these characters, as they are described so well.
It's free on Amazon if you have a kindle, and if you don't? You can set up an Amazon account and download an app for your PC and read it on your computer.