Is the famous book fanwank or the will of Roddenberry? Is the info in there taken from the show's bible, and, if so, is it canonical?
Yes, I know that strictly speaking, it has to be on the screen to be canon. But was the book sanctioned and meant to accurately represent the show and its background?
It accurately represents the thoughts and intentions of the show's producers at the time, at least as filtered through Whitfield's interpretations. Whitfield had full access to the production and got his information from a variety of production sources including the series bible, the original format proposal, countless production memos, interviews with Roddenberry and the staff, etc.
However, that's a totally different question from what is canon. Canon is the set of backstory assumptions observed by whoever's currently producing the show, and different producers make different assumptions. The material in TMoST is analogous to the material in a series bible -- it reflects the original intentions of the creators, but that doesn't preclude a later episode or film from contradicting those intentions for the sake of the story.
So even though TMoST represents the most official and authentic account of the way ST's producers perceived its characters and universe as of 1968, some of its assertions have been contradicted by later productions. For instance, it claimed Scotty was an only child, but TWOK gave him a nephew (though that relationship was cut from the theatrical version of the film so its canonicity is unclear).