I plan to sell my old Kindle Keyboard and I want to erase all the Kindle's data ( my books, my personal info, wi-fi passwords...) before. Is it enough to deregister the Kindle and execute a factory reset? Would it completely erase everything?
Every Kindle has a menu option "Reset to Factory Defaults", which does exactly what you want: It removes all user specific data from the device.
For the paranoid: Most devices will not actually erase your data, but just mark it as "erased". For the user both are the same thing - if you sell your device, the new owner will not see your stuff. For anybody with sufficient knowledge and interest in that piece of information - such as crime scene investigators etc. - it usually is possible to restore data. I have not checked if a Kindle will actually overwrite your data or leave traces behind.