Many excellent Etexts of out of copyright books can be found at the Project Gutenburg (http://promo.net/pg/).

You can also get some very good (free but in copyright) books from www.baen.com. Which also has links to other free online book sites (e.g. a couple of major university presses).

Some publishers make a few of their books available as etexts - but usually in a proprietry format and they charge (of course).

If you are after a specific book - no there is no reliable method of finding it online. You may, or may not, find what you are looking for with LimeWire.

Mark