I recently created my first .epub file. This is a type of e-book file that can be read on Android devices, the Nook, Sony Reader, and portable Apple devices among others. The files are a little odd in the way they need to be formatted. Here are a few resources I found helpful when creating my first document.
Calibre is a free and open source e-book library management application developed by users of e-books for users of e-books. This program allows you to convert between several different formats.
Sigil is Google’s free multi-platform WYSIWYG ebook editor.
Adobe TV ebook creation using InDesign
This is a link to a series of three videos on Adobe TV about ebook creation using InDesign. These are what I primarily used to get started, and they helped a lot, although you need a little bit of an InDesign background to get started. You also need to be using InDesign CS4 or CS5 to export the .epub file.