An author who self proclaims to tell the truth in a book that self proclaims to tell the truth is not believable. I could tell you the same thing and yet you would not believe me. Just because someone or something proclaims to be the truth and the word of god, does not make it so.
A selection of witnesses to Jesus' incarnate authority and power:
"When Jesus had finished saying these things, the crowds were amazed at his teaching, because he taught as one who had authority, and not as their teachers of the law." (Matthew 7:28-29 NIV)
"The people were all so amazed that they asked each other, 'What is this? A new teaching—and with authority! He even gives orders to impure spirits and they obey him." (Mark 1:27 NIV)
"One day as Jesus was teaching the people in the temple courts and proclaiming the good news, the chief priests and the teachers of the law, together with the elders, came up to him. 'Tell us by what authority you are doing these things,' they said. 'Who gave you this authority?' He replied, 'I will also ask you a question. Tell me: John’s baptism—was it from heaven, or of human origin?' They discussed it among themselves and said, 'If we say, ‘From heaven,’ he will ask, ‘Why didn’t you believe him?’ But if we say, 'Of human origin,' all the people will stone us, because they are persuaded that John was a prophet.' So they answered, 'We don’t know where it was from.' Jesus said, 'Neither will I tell you by what authority I am doing these things." (Luke 20:1-8 NIV)
"Rabbi, we know that you are a teacher who has come from God. For no one could perform the signs you are doing if God were not with him." (John 3:2 NIV)
"Jesus of Nazareth was a man accredited by God to you by miracles, wonders and signs, which God did among you through him, as you yourselves know." (Acts 2:22 NIV)