Strongly recommend treating them as two integral parts of one work of art -- consuming either one and not the other will leave you much less satisfied.
I think that may be true for several other longer adaptations including STALKER (from A Roadside Picnic) and Apocalypse Now (from A Heart of Darkness).
Tarkovsky had become increasingly disillusioned about the whole project as he entered into production — he had fights with his cinematographer and had doubts about the story. At one point, they had filmed 6 months of material when they discovered the film negatives were unusable, for reasons nobody has been able to figure out (they used Kodak stock imported from the US, which Soviet labs were unfamiliar with, and it may have been out of date; Tarkovsky himself suspected it was sabotage). As a result, the project went into hiatus while Tarkovsky considered his next steps and sought new funding from the Soviet film board to reshoot. It was actually Arkady Strugatsky, after many discussions and drafts, who suggested a compromise, where most of the sci-fi elements would be stripped from the film, and he encouraged Tarkovsky to build up the screenplay again based on his own ideas.
It's fascinating to think what Stalker might have been, if Tarkovsky hadn't deviated from the original script. Roadside Picnic is certainly a novel that could work as a film or miniseries. (WGN considered producing a TV show based on Roadside Picnic last year, with Matthew Goode as the lead, but the pilot was rejected.)
Perhaps the one book-then-movie reading/viewing sequence I'm happy with was Casino Royale. It was a good faithfully-gritty adaptation.