All the books were written by Buddha's disciples or were handed down Orally by the disciples to the writers. Now you may question the "authenticity" since it was handed down orally. But unlike other religions where books were very much subjective and not much care was given to any misrepresentations in the meanings, in buddhism the meanings were left closest as they could be to the original teaching.

Language has its limitations and Budhha being the OmniPotent of course knew that. Hence, the emphasis on your own reason/instinct.
I think it's a good idea to start with "premstha"'s quotes.
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"Do not believe in anything simply because you have heard it.
 Do not believe in anything simply because it is spoken and rumored by many.
 Do not believe in anything simply because it is found written in your religious books.
 Do not believe in anything merely on the authority of your teachers and elders.
 Do not believe in traditions because they have been handed down for many generations.
 But after observation and analysis,when you find that anything agrees with reason and is conducive to the good and benefit of one and  all, then accept it and live up to it."

 quotation of Buddha
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After analysis, one of the conclusions that can be drawn is that Budhha let You decide what you made of the different teachings. Very democratic indeed. But another way of looking at it is that, He has assumed that all human beings, if left on their own, will try to go the Right Way. Of course, that comes with Right Understanding and the Right Intentions.
I like that quote, Premstha, btw. Thanks for bringing back this quote to my mind.