There are elements of the film that I just chalked up to Tartovsky going off into some kind of dreamscape, with no context provided in the film. But the book provides that context and would make the film more coherent. Even with all that said, I'm not sure I'd have raved about the film even if I'd read the book first.

Jeff Vandemeer's Annihilation trilogy of books is a spin on Roadside Picnic that is also well worth reading (the second part being one of my favourite books of recent years). Totally unrelated to Stalker though, other than thematically.