I seriously doubt that any rule says that sources must be free. It helps in
double-checking some statements if other people can at least see the
abstract of an article, which is why
pubmed...gov is so good. Old books
might be better than new studies in some cases, and even when those are
public domain, they are not really free. Someone has to pay time for
scanning, or copying and supply web space, which is why project Gutenberg is
so good.
_______
Shaw's Principle:
Build a system that even a fool can use, and only a fool will want to use
it.
http://ecn.ab.ca/~brewhaha/WP_CRYSTAL.HTM