I didn't enjoy this book, simply because the author kept trying to sell that it was a true story but it was written like a kid trying to convince you of a lie
You could say the same thing about any work of fiction. The writer always tries to convince you that the events (s)he's describing really happened, even though you know they didn't, and (s)he knows you know they didn't, and you know (s)he knows you know they didn't.
Only apparently you don't. That's a little weird because the entire genre is based on the suspension of disbelief: you know the events described never happened but you try to believe, at least while the book is in your hands, that they really did.
Of course, for legal reasons, books sometimes carry a disclaimer that says: "This is a work of fiction. Any similarity to actual persons is purely coincidental …" but that's usually an indication that the characters in fact are thinly disguised portraits of real people. In other words, if they take the trouble to tell you it's a work of fiction, that's a sure-fire indication that it isn't.
I should point out that I don't know the writer but I enjoyed the book.
As to its credibility, I don't recall the main body of the book being any more or less credible than the lengthy free sample to which I provided a link. Or are you saying you read the free sample and would have been prepared at that time to swear on your granny's life that the events related therein really happened, and that it was only when you came to read the rest of the book that the doubts began to creep in?