suppose i publish a new 100-page book. is my book automatically copyrighted as long as i insert a notice on the front page? or do i ahve to go to a certain government agency?
Yes, when you've done something like this, you gain the copyright automatically. Another question that in case of dispute you will have to prove that the work is yours and you made it earlier (if you have not published it yet). That is why some people apply to organisations like the U.S. Copyright Office to register their works. Also, read the contract with the publisher carefully as you may involuntarily grant them your authorship and lose your copyright privileges.
letters is right, if you are in the US. This is what the US Copyright office has to say: http://www.copyright.gov/help/faq/faq-general.html#register
that cleared things up. thaks letters and nonny. another copyright question. if an author's copyright has expired, not renewed and thus has fallen into the public domain, is it legal for someone to use the work and leave out the author's name?
That's tricky. I believe that renewal hasn't been required since 1964, so that would only apply to earlier publications. Current works are copyrighted for the life of the author plus seventy years, so obviously none of those are in the public domain yet. http://www.copyright.gov/help/faq/faq-duration.html#duration There is useful chart here about when works fall into the public domain (scroll down): http://www.copyright.cornell.edu/training/Hirtle_Public_Domain.htm If you want to do a renewal search, there is information here: http://onlinebooks.library.upenn.edu/renewals.html http://www.copyright.gov/circs/circ22.pdf Note that there are no official transcriptions of the copyright renewals online (the copyright office only has records beginning in 1978 online). The only thing you can assume is that if the work was published before January 1, 1923, it's in the public domain. If it was published between 1923 and 1964, you will have to investigate.
When in dispute, especially if the burdent of proof is on you, it's certainly advantageous to be able to present prima facie evidence. For this reason, "automatic copyrighting" seems rather useless without the government recognizing your work.
Reading your question "verbatim" if public domain you have "fair use" but you have no right to claim copyright as everyone else also has "fair use". If your original is "substantially" your own work you can easily protect your claim. I bolded substantially since copyright law does not actually specify a percentage or word count thus open to interpretation.