For free electronic copies of public domain works, I have always loved Project Gutenberg.
For books in my phone, I like Books in my Phone.com (imagine that).
For audio books, though, the public domain recordings that I had previously found mostly sounded like a computer reading the book, like the Mac VoiceOver emulator.
Today I stumbled across LibriVox. It seems like decent enough public domain recordings. LibriVox consists of volunteers, so the quality is as good as the community wants it to be.
