If it's published and associated with a person, group, or organization, then yes, it'd be hella illegal. But, as it stands, you writing a book without being copyrighted means *everyone* wrote that book. In terms of who can use it, that is.
If you post a synopsis that's one thing, as the likelihood of someone copying the same exact story as yours is astronomical to basically impossible, but, still, be on guard when you post shit like that to those you're unfamiliar with.
