My research paper has been stolen on pretext of a book publication deal? What can I do?
This is an unfortunate situation. I would try to contact that person's current supervisor (if you can find out who) and explain all this, and maybe s/he can reprimand this person. Additionally, I would also contact the editor of the journal and explain the same to him/her. It might result in the paper retraction on grounds of plagiarism/academic dishonesty. However, I am not sure how would you be credited for the stolen work. Maybe an addendum saying it was your idea or maybe publishing it separately altogether.

Edit1: As @CaptainEmacs pointed out, "Add email date evidence. It is not perfect, but at least corroborates the dates."

Edit2: As @ASimpleAlgorithm pointed out, "Presumably there would be vastly more evidence possible in terms of prior revisions and incremental experimental or theoretical work. It only takes a very short conversation before it becomes obvious which person is the plagiarizer and which is the author."