the science fiction publisher, has a largish Free Library
of books whose authors are willing to let them be posted.
They also sell e-books, and sometimes give them away on CD's packaged with new hardcovers.
Eric Flint, author and librarian, claims
that the free samples produce a visible spike in sales.
Someone also pointed out
that the National Academies Press
" publishes all of its 2,500 books both in free online editions and in priced printed editions and, like you, has found that the free editions stimulate sales of the priced editions."