Its pretty clear the book had a negative slant towards Eisner and some others. But if you think that bias was pre-determined, its probably important to note that the author had Eisner's and the Company's cooperation in writing the book.
While its impossible to know what exactly was in the author's head at the beginning, its not a stretch at all to think that most of his slant came from what he found in doing his research. Certainly if Eisner thought it would be a "slam" piece, the company would not have cooperated. Its not like the author has a reputation for doing biased butcherings of his subjects.
Further, most of the "negativity" in the book is relayed from others, not the author's commentary.
Not to say its all absolutely true, but its about as unbiased a source as we have on most of this stuff.