Historians, so I have read, primarily rely on two types of sources in reconstructing the past. Primary and secondary. Primary sources are defined as:
In historiography, it is a document, recording or other source of information that was created at roughly the time being studied, by an authoritative source, usually one with direct personal knowledge of the events being described.
Primary sources are considered preferable because they offer a first-handed source from the past including diaries or artifacts that allows direct, unmediated information about the object of study.
I think McClellan's book would fall into that category. The problem with primary sources however is they have to be evaluated based on the sources biases. Historians use the following questions to do so:
What is the tone?
Who is the intended audience?
What is the purpose of the publication?
What assumptions does the author make?
What are the bases of the author's conclusions?
Does the author agree or disagree with other authors of the subject?
Does the content agree with what you know or have learned about the issue?
Where was the source made?
I think it is the first tow that will cause McClellan problems. I haven't read the book yet (and I don't know if I will) but the excerpts / snippets I have heard are so overwhelmingly hostile that I don't know how it can stand the test of time.
That's too bad. God knows the Bush administration has made a number of mistakes (one of the being having McClellan serve as Press Secretary for 3 years) but a reasoned examination of why the mistakes were made from McClellan's point of view probably would have been very enlightening vs. a mad dog attack.