I just got myself a free copy of The Adventures of Sherlock Holmes by Sir Arthur Conan Doyle. No strings attached and it was perfectly legal. You can get that book or thousands others like it for free too.
Project Gutenberg has the text of over 30,000 books for free to download.
For example you can get works from great authors such as Mark Twain or William Shakespeare among many other great authors. You can get books like Pride and Prejudice by Jane Austen or Ulysses by James Joyce just to name a couple.
The books come in multiple formats. You can get books in plain text, HTML or in ebook formats readable with Palm or many mobile phones. You can even get many of the books in audio book format.
Are they really free?
The books are those which are in the public domain, generally older books where the copyright has expired. I should note that while the copyright is expired for the books in the United States, the copyright may still be valid in other countries. If you're in a different country then you may want to check into the details.
How does it work?
Project Gutenberg is a non profit organization that uses volunteer labor to scan or transcript books into electronic format. They then proofread and post the material on their website. Its made available there for people to download for free. Project Gutenberg
is a non profit and you can donate your time or money to help support them.