As promised, director Oliver Stone and screenwriter Stanley Weiter have posted a webpage of annotations and footnotes for practically every scene in W., revealing their sources for what the movie shows us.
The page is fun to click through even if you haven't seen the film. For example, as proof of the movie's depiction of President Bush imbibing non-alcoholic beer, the filmmakers offer the following quote from the 2003 book Bush's Brain by James Moore and Wayne Slater:
[Speaking about his 2000 Presidential Election]… “‘I’ve won,’ said George W. Bush, one week before Election Day. A couple of reporters on the plane appeared unconvinced. But Bush was supremely confident, leaning against the bulkhead with a Buckler near-beer in his hand…”