iBooks needs wi-fi to download bigger books, but once they are downloaded you can read them offline. Kindle for Amazon.com books also downloads the books for offline reading.
For iBooks, you just go to the Store in the app, find the book you want, and download (free or paid). Once the book finishes downloading you can find and read it under the Library page.
For Kindle you need to purchase the book at Amazon.com first. Then you download the Kindle app and register it with the same account you purchased the book with. All books you've purchased will show up on the Cloud screen. Tap them and they download to the device screen for reading.
These are the two biggest sources for books on the iPad. There is also a Nook app for Barnes and Noble books. It works similarly to the Kindle app.