What does the expression "tilting at windmills" mean?

And has anyone read the book Tilting at Windmills by Julian Branston? If so, why does Denia spread the rumour that Cervantes is being considered for the court poet after the emperor dies?
Update: Thank you exaybachay. I'm actually doing a book report on this book and didn't know much about the character of Don Quixote. I'm pretty sure the entire book is a mockery of Don Quixote because in the book, there is this senile old man who "becomes a knight" and he wears his armor upsidedown... show more Thank you exaybachay. I'm actually doing a book report on this book and didn't know much about the character of Don Quixote. I'm pretty sure the entire book is a mockery of Don Quixote because in the book, there is this senile old man who "becomes a knight" and he wears his armor upsidedown and rides his horse backwards.
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