A more serious post from me, this time.
I wonder what the position would be if your book appeared on the Web in its complete, if draft, form. Would it be seen to have been published? What redress do you have? What would the opinion of potential agents and publishers be?
I know that these are unlikely consequences of mailing the document to this man, but I'd like to know what those more experienced with the industry think about this.
This is a very good question. It
would be considered a published work, which would not be helpful in selling the manuscript to a publisher. How long it was actually on the web might factor in, though. (If you found out within a few days and made him remove it, for instance.)
What redress you might have -- you could sue the person responsible, but a court would probably have a hard time figuring out the damages on an unpublished manuscript, since no value has been established.
The best protection against something like this happening is that no one would have much to gain by doing it. That doesn't mean that there isn't somebody somewhere in the world who would put another person's work on the web for the ego gratification of claiming it, however briefly, as his or her own. Yes, you could ultimately prove that it was yours, but only after a lot of trouble and stress and expense.
I, personally, would be wary about sending an electronic file of an entire book to someone I didn't know very well, someone who hadn't a professional reputation to maintain. When you send something to a publisher or an agent, their ability to go on doing business depends on maintaining the public's trust; they stand to lose far more by stealing a writer's work than they are likely gain by it. This would not be the case here. Miniscule as the chances are that he would steal from you, are the benefits you might expect to gain by sending it to him enough to offset even that tiny risk? Perhaps you've gained valuable insights from his critique of the part he's already seen. The possibility of gaining more valuable input would definitely be something to weigh in the balance. But if you are thinking about sending it to him mostly because you are flattered that he asked, I don't think that should weigh in favor of sending it.