A few things to ease your mind. (The following information refers to US Copyrights.)
First, you took the correct action in contacting the webmaster of the web site. If the work were passed along without crediting you, it could fall into the public domain, which means anyone could use it in any way and without crediting you. Challenging an infringement is the correct move. Basically, the webmaster has now been alerted that he is in possession of stolen goods. Keep a record of your communication with him.
Second, you may be aware already that, by US copyright law, as soon as you create a work you have the legal copyright. A copyright registration for a nominal fee registers a work with the US Copyright Office, but it's not necessary for protection; it's just an additional measure, a record of your copyright of a work. You may go online and secure a copyright for each work, but it is best to put several of your works together and register their copyright as a "collection." It saves time and money.
Third, contrary to what most new creators believe, the best thing to do with new works is to expose them to as many non-relative witnesses as possible. By publishing your work here on MS, you have an international group of witnesses, with verifiable evidence of the first publishing calendar dates of your works. Yes, putting your work out there on a blog or web site falls under the copyright office's definition of "publishing."
Fourth, YOU DO NOT NEED TO HIRE ASSISTANCE in registering your copyrights. Anyone intelligent enough to create a work is intelligent enough to fill out the registration paperwork.
Last, there is a popular and false rumor that one can mail himself a copy of his work and it will serve as evidence in a copyright dispute. This is false. Mailing one's self his own work has no merit in a court of copyright law.
Rest easy. In following your instincts you have made EXCELLENT moves. This may be a good time to begin registering your copyrights, as your work has proved to be worth plagiarizing.
Simply visit: United States Copyright Office
I'll look forward to buying a copy of your collection at Barnes & Noble!