Magento offer a paid ebook User Guide (which costs around USD$20). What a lot of people don't know is that you will find the official and free version of the Magento User Guide inside the official Magento website, but it is almost impossible to find and does not show in any Google search.
Here is the link to the official and free online user guide:
http://www.magentocommerce.com/wiki/welcome_to_the_magento_user_s_guide/welcome_to_the_magento_user_s_guide
