Tim Harding's The Kibitzer column on Chesscafe.com recently had an article about finding old chess books online (http://www.chesscafe.com/text/kibitz197.pdf). Basically he goes to Google Books and Forgotten Books, and searches for chess. See the article for details.
In the article, he says he's found the following old chess books this way, although whether they are available to you or not depends on your country:
- The exploits and triumphs, in Europe, of Paul Morphy, by Morphy's
late secretary (Edge). The English edition, with a slightly different title,
is probably also available.
- A Popular Introduction to the Study and Practice of Chess…by an
Amateur (1851). This is an early manual that came out soon after
Staunton's Handbook. The author was S. S. Boden, a leading
contemporary of Bird and Morphy.
- Chess Brilliants, One Hundred Games, selected by I. O. Howard
Taylor of Norwich Chess Club (1869)
- The Chess-Player's Annual for the Year 1856, edited by Charles
Tomlinson (the only year that ever came out, but full of interesting
material). Also his Amusements in Chess.
- Staunton's Chess Praxis, and other books by him.
- Several George Walker books, including Chess Studies (the first large
published collection of actual play, over 1,000 games) and his Chess
and Chess Players, still a good read, which includes the articles he
wrote for Fraser's Magazine. Also his collection of Games Actually
Played by Philidor should be available.
- Horae Divanianae, by Elijah Williams (1852), a collection of 150
games played at the Grand Divan in the Strand.
- Willard Fiske's The book of the first American chess congress is well
- The Principles of Chess in Theory & Practice by James Mason (1894);
this is one I got from Forgotten Books.
- Some early books on chess compositions are also available, for
example A collection of two hundred chess problems, by Francis
Edit: Now I see that this doesn't really answer your question. Perhaps see here on Wikipedia to see how long copyright in the book's author's country lasts, to find out whether a book is in the public domain.