Here is all the documentation for Nikola. Note that the documents may not always be up-to-date. If you want to read the documentation for the current GitHub master, check out Nikola on ReadTheDocs.

The Handbook
Aimed at those wanting to use Nikola to build a site.
Upgrading to Nikola v6
Useful if you were using version 5.
Theming Tutorial and Theming reference
Useful if you want to create or customize a theme.
Extending Nikola and Nikola Internals
Aimed at those wanting to add new functionality.
Creating a Site that is not a blog
Not everything is a blog!
Using Social Buttons
Replacing AddThis with other solutions.
Tutorial by Tim van der Linden
Goes from installing to developing a custom theme.
Tutorial en español de Diego Leonardo Puente
Instalación y deployment a GitHub
Tutorial by RCA (in Chinese)
Covers installation and getting started
Wordpress migration tutorial by Niko Wenselowski (in German)
He wrote most of the Wordpress importer so he knows ;-)
Moving from Octopress to Nikola
Since we don't have an automated tool...