Relatively little of the general copyright information posted to the internet is directly relevant to the situation of an historian or biographer or text editor working with manuscript materials. Such as there is:
The text of the Copyright Act (1976, as emended 1994), Title 17 USC, as available from the House gopher or in HTML format from Cornell, is the quaking rock upon which all further discussion is founded.
Also handy is the Copyright Office's Circular 1, Copyright Basics also available from CoOL.
For textual scholars dealing with unpublished manuscripts, the most important sections of the Act are 17 USC 104 and 17 USC 107
Terry Carroll's Copyright FAQ is available in both gopher and HTML forms. Terry, a mainstay of CNI-COPYRIGHT, author of the comp.fonts FAQ, law student and lawyer, is an incisive thinker and pungent writer well worth consulting on almost any question.
Much of the best practical information on copyright as used and suffered by scholars and librarians is to be found in the various archived lists. Most reliable (though always to be taken skeptically, of course) is CNI-COPYRIGHT. Also useful, because of its concentration of humanists experienced with the copyright difficulties attendant on electronic texts, is the original humanities list, HUMANIST. Both lists are archived.
Additional law-related discussion groups can be discovered via two lists, one at Cornell, one at Kent State.
For photographic questions, consult the Copyright Guide For Photographers, produced by the American Society of Media Photographers, Inc.
The critical Feist decision may be found at the Cornell archive, along with a searchable index of Supreme Court opinions and a Searchable index of Second Circuit Court decisions.