... and why is it relevant to Kate’s Book?
The inference is that poor suffering Kate McCann wrote her book purely to raise funds to continue the search for her missing daughter. Despite her natural reserve and desire for privacy she shared details about her life for Madeleine's sake, not for any other reason.
The Fund needed the money and Kate donated every penny the book earned, we are told. We don't know how much she earned, of course, or how much of it was donated. Those details haven't been shared. We also don't know why she felt the need to place a restriction on the use of the money. Although she's a director of the Fund she seems to have felt the need to restrict the use to which the book earnings were/are put. Does she not trust her fellow directors to spend the money wisely?
On the other hand, of course, nasty vindictive Amaral wrote a book and kept the money for himself, just as many others did. The difference was that he wrote about the investigation into Madeleine's disappearance rather than writing a book defending her parents.