Why would the speaker mention "write the book" in this context?

Native speaker’s answer
Rebecca
It's a metaphorical book! There's a common phrase, "to do things by the book," and that means to do things the way they're supposed to be done; how they're expected to be done. However! In this context, they're saying they get to choose how they do things and the way they want them to be done because of how skilled and good they are, even if they are entry-level. Ex: I don't go by the book. I write the book. Ex: We always do things by the book when it comes to accounting.