quarantine 'em

The thing which struck me about The Martian was that the film is an extremely faithful adaptation of the book. As were The Hunger Games and The Social Network

This wasn't always the case. Time was, a director would get bored less than a hundred pages into your book and then make the rest up

This is a really good thing from the perspective of book authors about to get film deals. Of course it says nothing about film quality or book quality

Is it actually good for book authors? I'd agree they probably prefer it, but (remaining agnostic on the quality outlook) they might be wrong about what is good for them.

I think book authors generally like it if people who watch the film first, when they read the book, don't react with "What the heck, this is completely alien to me" and stop reading

well sure but this is part of a complex bundled good, the film, and it is probably more important that the film be good. the very existence of these hypothetical offended readers is endogenous to whether the film is good.