I don’t like Chelsea Clinton, but I support her in this. Ideas for books aren’t copyrighted, as far as I know. Did he write a book? Or did he just tell her his idea for a book? Here’s a lesson for him: don’t trust a Clinton.
Actually, if she can’t demonstrate she had the idea first or at the same time, it is theft of intellectual property. If he sent in a prospectus for the book and she copied it, she owes money.
You misunderstand how book contracts are done. First the author sends a detailed description of a book proposal with a sample of the writing (but ususally does not send the entire book, to prevent copying). Second, the publisher negotiates a contract with the person who submitted the idea. Third, the book is then written, illustrated, edited, printed and distributed in stages.
Because the first part is so risky and the third part is so expensive, the process has evolved over the years to prevent losses on either side. So if the publisher did, in fact, share the idea with Chelsea in order to hang a "celebrity progressive" name on the idea to increase profits, she or he did an unethical thing.
I don't really know whether the Facebook submission of the idea will stand up as well as a formal cover letter with a paper manuscript and a mailing date, so the judge may break new ground if the FB submission is given the same weight.
Nevertheless, I'm not expecting a win against the Clintons.